diff --git a/src/cairo-xlib-surface.c b/src/cairo-xlib-surface.c
index 321857d..2b5c4fe 100644
--- a/src/cairo-xlib-surface.c
+++ b/src/cairo-xlib-surface.c
@@ -627,6 +627,10 @@ _get_image_surface (cairo_xlib_surface_t    *surface,
 	masks.red_mask = surface->visual->red_mask;
 	masks.green_mask = surface->visual->green_mask;
 	masks.blue_mask = surface->visual->blue_mask;
+	if (surface->depth == 32)
+	    masks.alpha_mask = ~(masks.red_mask   |
+		                 masks.green_mask |
+		                 masks.blue_mask);
     } else if (surface->xrender_format) {
 	masks.bpp = ximage->bits_per_pixel;
 	masks.red_mask = (unsigned long)surface->xrender_format->direct.redMask << surface->xrender_format->direct.red;