diff --git a/contrib/pngminus/pnm2png.c b/contrib/pngminus/pnm2png.c
index 2298136..0d2caef 100644
--- a/contrib/pngminus/pnm2png.c
+++ b/contrib/pngminus/pnm2png.c
@@ -392,7 +392,7 @@
     return FALSE;
   }
   if ((png_pixels = (png_byte *)
-     malloc (row_bytes * height * sizeof (png_byte))) == NULL)
+     malloc ((size_t)row_bytes * (size_t)height * sizeof (png_byte))) == NULL)
     return FALSE;
 
   /* read data from PNM file */