diff --git a/libtiff/tif_pixarlog.c b/libtiff/tif_pixarlog.c
index 77620e3d..6998c1ba 100644
--- a/libtiff/tif_pixarlog.c
+++ b/libtiff/tif_pixarlog.c
@@ -640,6 +640,7 @@ PixarLogGuessDataFmt(TIFFDirectory *td)
 static tmsize_t
 multiply_ms(tmsize_t m1, tmsize_t m2)
 {
+        assert(m1 >= 0 && m2 >= 0);
         if( m1 == 0 || m2 > TIFF_TMSIZE_T_MAX / m1 )
             return 0;
         return m1 * m2;
@@ -648,6 +649,7 @@ multiply_ms(tmsize_t m1, tmsize_t m2)
 static tmsize_t
 add_ms(tmsize_t m1, tmsize_t m2)
 {
+        assert(m1 >= 0 && m2 >= 0);
 	/* if either input is zero, assume overflow already occurred */
 	if (m1 == 0 || m2 == 0)
 		return 0;