diff --git a/code/CMakeLists.txt b/code/CMakeLists.txt
index d2c59d756..e305245bd 100644
--- a/code/CMakeLists.txt
+++ b/code/CMakeLists.txt
@@ -1245,7 +1245,6 @@ IF (ASSIMP_WARNINGS_AS_ERRORS)
         -Wno-deprecated-copy-with-dtor
         -Wno-deprecated
         -Wno-format-nonliteral
-        -Wno-format-non-iso
         -Wno-comma
         -Wno-unreachable-code-break
         -Wno-unreachable-code-return
diff --git a/include/assimp/StringUtils.h b/include/assimp/StringUtils.h
index 59c6e9ead..cd8726785 100644
--- a/include/assimp/StringUtils.h
+++ b/include/assimp/StringUtils.h
@@ -56,7 +56,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include <sstream>
 #include <iomanip>
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 #define AI_SIZEFMT "%Iu"
 #else
 #define AI_SIZEFMT "%zu"