diff --git a/code/AssetLib/3DS/3DSConverter.cpp b/code/AssetLib/3DS/3DSConverter.cpp
index c977867c0..aca16b0d6 100644
--- a/code/AssetLib/3DS/3DSConverter.cpp
+++ b/code/AssetLib/3DS/3DSConverter.cpp
@@ -69,7 +69,7 @@ void Discreet3DSImporter::ReplaceDefaultMaterial() {
     for (unsigned int i = 0; i < mScene->mMaterials.size(); ++i) {
         std::string s = mScene->mMaterials[i].mName;
         for (std::string::iterator it = s.begin(); it != s.end(); ++it) {
-            *it = static_cast<char>(::tolower(*it));
+            *it = static_cast<char>(::tolower(static_cast<unsigned char>(*it)));
         }
 
         if (std::string::npos == s.find("default")) continue;
diff --git a/code/AssetLib/AMF/AMFImporter.cpp b/code/AssetLib/AMF/AMFImporter.cpp
index 1a3efba9a..e77b65f77 100644
--- a/code/AssetLib/AMF/AMFImporter.cpp
+++ b/code/AssetLib/AMF/AMFImporter.cpp
@@ -205,7 +205,7 @@ void AMFImporter::ParseHelper_FixTruncatedFloatString(const char *pInStr, std::s
 }
 
 static bool ParseHelper_Decode_Base64_IsBase64(const char pChar) {
-    return (isalnum(pChar) || (pChar == '+') || (pChar == '/'));
+    return (isalnum((unsigned char)pChar) || (pChar == '+') || (pChar == '/'));
 }
 
 void AMFImporter::ParseHelper_Decode_Base64(const std::string &pInputBase64, std::vector<uint8_t> &pOutputData) const {
diff --git a/code/AssetLib/BVH/BVHLoader.cpp b/code/AssetLib/BVH/BVHLoader.cpp
index 8ae99033c..cf78fb6c6 100644
--- a/code/AssetLib/BVH/BVHLoader.cpp
+++ b/code/AssetLib/BVH/BVHLoader.cpp
@@ -359,7 +359,7 @@ void BVHLoader::ReadMotion(aiScene * /*pScene*/) {
 std::string BVHLoader::GetNextToken() {
     // skip any preceding whitespace
     while (mReader != mBuffer.end()) {
-        if (!isspace(*mReader))
+        if (!isspace((unsigned char)*mReader))
             break;
 
         // count lines
@@ -372,7 +372,7 @@ std::string BVHLoader::GetNextToken() {
     // collect all chars till the next whitespace. BVH is easy in respect to that.
     std::string token;
     while (mReader != mBuffer.end()) {
-        if (isspace(*mReader))
+        if (isspace((unsigned char)*mReader))
             break;
 
         token.push_back(*mReader);
diff --git a/code/AssetLib/Blender/BlenderLoader.cpp b/code/AssetLib/Blender/BlenderLoader.cpp
index 3722b9c73..56f4e985f 100644
--- a/code/AssetLib/Blender/BlenderLoader.cpp
+++ b/code/AssetLib/Blender/BlenderLoader.cpp
@@ -420,9 +420,9 @@ void BlenderImporter::ResolveImage(aiMaterial *out, const Material *mat, const M
             --s;
         }
 
-        curTex->achFormatHint[0] = s + 1 > e ? '\0' : (char)::tolower(s[1]);
-        curTex->achFormatHint[1] = s + 2 > e ? '\0' : (char)::tolower(s[2]);
-        curTex->achFormatHint[2] = s + 3 > e ? '\0' : (char)::tolower(s[3]);
+        curTex->achFormatHint[0] = s + 1 > e ? '\0' : (char)::tolower((unsigned char)s[1]);
+        curTex->achFormatHint[1] = s + 2 > e ? '\0' : (char)::tolower((unsigned char)s[2]);
+        curTex->achFormatHint[2] = s + 3 > e ? '\0' : (char)::tolower((unsigned char)s[3]);
         curTex->achFormatHint[3] = '\0';
 
         // tex->mHeight = 0;
diff --git a/code/AssetLib/Collada/ColladaParser.cpp b/code/AssetLib/Collada/ColladaParser.cpp
index 42166fdd4..1ef109f11 100644
--- a/code/AssetLib/Collada/ColladaParser.cpp
+++ b/code/AssetLib/Collada/ColladaParser.cpp
@@ -234,7 +234,7 @@ void ColladaParser::UriDecodePath(aiString &ss) {
 #if defined(_MSC_VER)
     if (ss.data[0] == '/' && isalpha((unsigned char)ss.data[1]) && ss.data[2] == ':') {
 #else
-    if (ss.data[0] == '/' && isalpha(ss.data[1]) && ss.data[2] == ':') {
+    if (ss.data[0] == '/' && isalpha((unsigned char)ss.data[1]) && ss.data[2] == ':') {
 #endif
         --ss.length;
         ::memmove(ss.data, ss.data + 1, ss.length);
diff --git a/code/AssetLib/FBX/FBXMaterial.cpp b/code/AssetLib/FBX/FBXMaterial.cpp
index 3af014bc3..409d7304a 100644
--- a/code/AssetLib/FBX/FBXMaterial.cpp
+++ b/code/AssetLib/FBX/FBXMaterial.cpp
@@ -82,7 +82,7 @@ Material::Material(uint64_t id, const Element& element, const Document& doc, con
 
     // lower-case shading because Blender (for example) writes "Phong"
     for (size_t i = 0; i < shading.length(); ++i) {
-        shading[i] = static_cast<char>(tolower(shading[i]));
+        shading[i] = static_cast<char>(tolower(static_cast<unsigned char>(shading[i])));
     }
     std::string templateName;
     if(shading == "phong") {
diff --git a/code/AssetLib/M3D/M3DImporter.cpp b/code/AssetLib/M3D/M3DImporter.cpp
index b7ba74c8a..1fb21d73e 100644
--- a/code/AssetLib/M3D/M3DImporter.cpp
+++ b/code/AssetLib/M3D/M3DImporter.cpp
@@ -136,7 +136,7 @@ bool M3DImporter::CanRead(const std::string &pFile, IOSystem *pIOHandler, bool c
         */
         std::unique_ptr<IOStream> pStream(pIOHandler->Open(pFile, "rb"));
         unsigned char data[4];
-        if (4 != pStream->Read(data, 1, 4)) {
+        if (!pStream || 4 != pStream->Read(data, 1, 4)) {
             return false;
         }
         return !memcmp(data, "3DMO", 4) /* bin */
diff --git a/code/AssetLib/MD3/MD3Loader.cpp b/code/AssetLib/MD3/MD3Loader.cpp
index e27079766..3002aff67 100644
--- a/code/AssetLib/MD3/MD3Loader.cpp
+++ b/code/AssetLib/MD3/MD3Loader.cpp
@@ -702,7 +702,7 @@ void MD3Importer::InternReadFile(const std::string &pFile, aiScene *pScene, IOSy
     }
     filename = mFile.substr(s), path = mFile.substr(0, s);
     for (std::string::iterator it = filename.begin(); it != filename.end(); ++it) {
-        *it = static_cast<char>(tolower(*it));
+        *it = static_cast<char>(tolower(static_cast<unsigned char>(*it)));
     }
 
     // Load multi-part model file, if necessary
diff --git a/code/AssetLib/X/XFileImporter.cpp b/code/AssetLib/X/XFileImporter.cpp
index cb245ed74..1fcd6b3db 100644
--- a/code/AssetLib/X/XFileImporter.cpp
+++ b/code/AssetLib/X/XFileImporter.cpp
@@ -667,8 +667,8 @@ void XFileImporter::ConvertMaterials( aiScene* pScene, std::vector<XFile::Materi
 
                 // convert to lower case for easier comparison
                 for ( unsigned int c = 0; c < sz.length(); ++c ) {
-                    if ( isalpha( sz[ c ] ) ) {
-                        sz[ c ] = (char) tolower( sz[ c ] );
+                    if ( isalpha( (unsigned char) sz[ c ] ) ) {
+                        sz[ c ] = (char) tolower( (unsigned char) sz[ c ] );
                     }
                 }
 
diff --git a/code/AssetLib/X/XFileParser.cpp b/code/AssetLib/X/XFileParser.cpp
index c5ce426ae..494ef5b2b 100644
--- a/code/AssetLib/X/XFileParser.cpp
+++ b/code/AssetLib/X/XFileParser.cpp
@@ -1245,13 +1245,13 @@ unsigned int XFileParser::ReadInt() {
         }
 
         // at least one digit expected
-        if (!isdigit(*mP))
+        if (!isdigit((unsigned char)*mP))
             ThrowException("Number expected.");
 
         // read digits
         unsigned int number = 0;
         while (mP < mEnd) {
-            if (!isdigit(*mP))
+            if (!isdigit((unsigned char)*mP))
                 break;
             number = number * 10 + (*mP - 48);
             mP++;
diff --git a/code/Common/BaseImporter.cpp b/code/Common/BaseImporter.cpp
index 2783a8372..02aa47326 100644
--- a/code/Common/BaseImporter.cpp
+++ b/code/Common/BaseImporter.cpp
@@ -179,7 +179,7 @@ void BaseImporter::GetExtensionList(std::set<std::string> &extensions) {
         }
 
         for (size_t i = 0; i < read; ++i) {
-            buffer[i] = static_cast<char>(::tolower(buffer[i]));
+            buffer[i] = static_cast<char>(::tolower((unsigned char)buffer[i]));
         }
 
         // It is not a proper handling of unicode files here ...
@@ -200,7 +200,7 @@ void BaseImporter::GetExtensionList(std::set<std::string> &extensions) {
             token.clear();
             const char *ptr(tokens[i]);
             for (size_t tokIdx = 0; tokIdx < len; ++tokIdx) {
-                token.push_back(static_cast<char>(tolower(*ptr)));
+                token.push_back(static_cast<char>(tolower(static_cast<unsigned char>(*ptr))));
                 ++ptr;
             }
             const char *r = strstr(buffer, token.c_str());
@@ -209,7 +209,7 @@ void BaseImporter::GetExtensionList(std::set<std::string> &extensions) {
             }
             // We need to make sure that we didn't accidentially identify the end of another token as our token,
             // e.g. in a previous version the "gltf " present in some gltf files was detected as "f "
-            if (noAlphaBeforeTokens && (r != buffer && isalpha(r[-1]))) {
+            if (noAlphaBeforeTokens && (r != buffer && isalpha(static_cast<unsigned char>(r[-1])))) {
                 continue;
             }
             // We got a match, either we don't care where it is, or it happens to
diff --git a/contrib/draco/src/draco/io/parser_utils.cc b/contrib/draco/src/draco/io/parser_utils.cc
index 753a1b314..4f95f6f84 100644
--- a/contrib/draco/src/draco/io/parser_utils.cc
+++ b/contrib/draco/src/draco/io/parser_utils.cc
@@ -252,7 +252,7 @@ DecoderBuffer ParseLineIntoDecoderBuffer(DecoderBuffer *buffer) {
 
 std::string ToLower(const std::string &str) {
   std::string out;
-  std::transform(str.begin(), str.end(), std::back_inserter(out), tolower);
+  std::transform(str.begin(), str.end(), std::back_inserter(out), [](unsigned char c){return tolower(c);});
   return out;
 }
 
diff --git a/contrib/draco/src/draco/io/ply_reader.cc b/contrib/draco/src/draco/io/ply_reader.cc
index ea7f2689a..cb32df225 100644
--- a/contrib/draco/src/draco/io/ply_reader.cc
+++ b/contrib/draco/src/draco/io/ply_reader.cc
@@ -268,14 +268,14 @@ std::vector<std::string> PlyReader::SplitWords(const std::string &line) {
   while ((end = line.find_first_of(" \t\n\v\f\r", start)) !=
          std::string::npos) {
     const std::string word(line.substr(start, end - start));
-    if (!std::all_of(word.begin(), word.end(), isspace)) {
+    if (!std::all_of(word.begin(), word.end(), [](unsigned char c){return isspace(c);})) {
       output.push_back(word);
     }
     start = end + 1;
   }
 
   const std::string last_word(line.substr(start));
-  if (!std::all_of(last_word.begin(), last_word.end(), isspace)) {
+  if (!std::all_of(last_word.begin(), last_word.end(), [](unsigned char c){return isspace(c);})) {
     output.push_back(last_word);
   }
   return output;
diff --git a/contrib/gtest/include/gtest/internal/gtest-param-util.h b/contrib/gtest/include/gtest/internal/gtest-param-util.h
index 82cab9b02..9d725a433 100644
--- a/contrib/gtest/include/gtest/internal/gtest-param-util.h
+++ b/contrib/gtest/include/gtest/internal/gtest-param-util.h
@@ -644,7 +644,7 @@ class ParameterizedTestCaseInfo : public ParameterizedTestCaseInfoBase {
 
     // Check for invalid characters
     for (std::string::size_type index = 0; index < name.size(); ++index) {
-      if (!isalnum(name[index]) && name[index] != '_')
+      if (!IsAlNum(name[index]) && name[index] != '_')
         return false;
     }
 
diff --git a/include/assimp/ParsingUtils.h b/include/assimp/ParsingUtils.h
index b52b0610e..b5074869e 100644
--- a/include/assimp/ParsingUtils.h
+++ b/include/assimp/ParsingUtils.h
@@ -262,7 +262,7 @@ AI_FORCE_INLINE unsigned int tokenize(const string_type &str, std::vector<string
 inline std::string ai_stdStrToLower(const std::string &str) {
     std::string out(str);
     for (size_t i = 0; i < str.size(); ++i) {
-        out[i] = (char) tolower(out[i]);
+        out[i] = (char) tolower((unsigned char)out[i]);
     }
     return out;
 }
diff --git a/include/assimp/StringComparison.h b/include/assimp/StringComparison.h
index 6dad01294..b0161664f 100644
--- a/include/assimp/StringComparison.h
+++ b/include/assimp/StringComparison.h
@@ -146,8 +146,8 @@ inline int ASSIMP_stricmp(const char *s1, const char *s2) {
 #else
     char c1, c2;
     do {
-        c1 = tolower(*s1++);
-        c2 = tolower(*s2++);
+        c1 = tolower((unsigned char)*(s1++));
+        c2 = tolower((unsigned char)*(s2++));
     } while (c1 && (c1 == c2));
     return c1 - c2;
 #endif
@@ -197,8 +197,8 @@ inline int ASSIMP_strincmp(const char *s1, const char *s2, unsigned int n) {
     unsigned int p = 0;
     do {
         if (p++ >= n) return 0;
-        c1 = tolower(*s1++);
-        c2 = tolower(*s2++);
+        c1 = tolower((unsigned char)*(s1++));
+        c2 = tolower((unsigned char)*(s2++));
     } while (c1 && (c1 == c2));
 
     return c1 - c2;
diff --git a/include/assimp/StringUtils.h b/include/assimp/StringUtils.h
index d536b2f0a..e56bee5d5 100644
--- a/include/assimp/StringUtils.h
+++ b/include/assimp/StringUtils.h
@@ -157,7 +157,7 @@ AI_FORCE_INLINE std::string ai_decimal_to_hexa(T toConvert) {
     ss >> result;
 
     for (size_t i = 0; i < result.size(); ++i) {
-        result[i] = (char)toupper(result[i]);
+        result[i] = (char)toupper((unsigned char)result[i]);
     }
 
     return result;
diff --git a/tools/assimp_view/Material.cpp b/tools/assimp_view/Material.cpp
index 8141fb58c..bcc93011e 100644
--- a/tools/assimp_view/Material.cpp
+++ b/tools/assimp_view/Material.cpp
@@ -272,7 +272,7 @@ bool CMaterialManager::TryLongerPath(char* szTemp,aiString* p_szString)
                             szExtFound - 1 - info.cFileName);
 
                         for (unsigned int i = 0; i < iSizeFound;++i)
-                            info.cFileName[i] = (CHAR)tolower(info.cFileName[i]);
+                            info.cFileName[i] = (CHAR)tolower((unsigned char)info.cFileName[i]);
 
                         if (0 == memcmp(info.cFileName,szFile2, std::min(iSizeFound,iSize)))
                         {
@@ -354,7 +354,7 @@ int CMaterialManager::FindValidPath(aiString* p_szString)
             for (unsigned int i = 0;;++i)
             {
                 if ('\0' == szTemp[i])break;
-                szTemp[i] = (char)tolower(szTemp[i]);
+                szTemp[i] = (char)tolower((unsigned char)szTemp[i]);
             }
 
             if(TryLongerPath(szTemp,p_szString))return 1;