diff --git a/pixman/pixman-edge.c b/pixman/pixman-edge.c
index 8d498ab..ad6dfc4 100644
--- a/pixman/pixman-edge.c
+++ b/pixman/pixman-edge.c
@@ -374,6 +374,7 @@ pixman_rasterize_edges (pixman_image_t *image,
                         pixman_fixed_t  b)
 {
     return_if_fail (image->type == BITS);
+    return_if_fail (PIXMAN_FORMAT_TYPE (image->bits.format) == PIXMAN_TYPE_A);
     
     if (image->bits.read_func || image->bits.write_func)
 	pixman_rasterize_edges_accessors (image, l, r, t, b);
diff --git a/pixman/pixman-trap.c b/pixman/pixman-trap.c
index ab5c8c8..4dad179 100644
--- a/pixman/pixman-trap.c
+++ b/pixman/pixman-trap.c
@@ -491,6 +491,8 @@ pixman_composite_trapezoids (pixman_op_t		op,
 {
     int i;
 
+    return_if_fail (PIXMAN_FORMAT_TYPE (mask_format) == PIXMAN_TYPE_A);
+    
     if (n_traps <= 0)
 	return;