diff --git a/fb/fbmmx.c b/fb/fbmmx.c
index 0e9074648..cea8ad9d5 100644
--- a/fb/fbmmx.c
+++ b/fb/fbmmx.c
@@ -1900,7 +1900,7 @@ fbCompositeSolidMaskSrc_nx8x8888mmx (CARD8      op,
 	    }
 	    else
 	    {
-		*dst = 0;
+		*(ullong *)dst = 0;
 	    }
 
 	    mask += 2;