diff --git a/code/M3D/M3DMaterials.h b/code/M3D/M3DMaterials.h
index 86a802021..b3c91ab7a 100644
--- a/code/M3D/M3DMaterials.h
+++ b/code/M3D/M3DMaterials.h
@@ -54,7 +54,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  * only once. D.R.Y. and K.I.S.S.
  */
 typedef struct {
-    char *pKey;
+    const char *pKey;
     unsigned int type;
     unsigned int index;
 } aiMatProp;