diff --git a/libfprint/nbis/mindtct/maps.c b/libfprint/nbis/mindtct/maps.c
index 239005f..ac3b82d 100644
--- a/libfprint/nbis/mindtct/maps.c
+++ b/libfprint/nbis/mindtct/maps.c
@@ -677,6 +677,7 @@ int morph_TF_map(int *tfmap, const int mw, const int mh,
    int *mptr;
    int i;
 
+   ASSERT_INT_MUL(mw, mh);
 
    /* Convert TRUE/FALSE map into a binary byte image. */
    cimage = (unsigned char *)malloc(mw*mh);