diff --git a/gfx/cairo/cairo/src/cairo-quartz-surface.c b/gfx/cairo/cairo/src/cairo-quartz-surface.c
--- a/gfx/cairo/cairo/src/cairo-quartz-surface.c
+++ b/gfx/cairo/cairo/src/cairo-quartz-surface.c
@@ -436,16 +436,17 @@ _cairo_quartz_cairo_operator_to_quartz_c
 	case CAIRO_OPERATOR_DIFFERENCE:
 	case CAIRO_OPERATOR_EXCLUSION:
 	case CAIRO_OPERATOR_HSL_HUE:
 	case CAIRO_OPERATOR_HSL_SATURATION:
 	case CAIRO_OPERATOR_HSL_COLOR:
 	case CAIRO_OPERATOR_HSL_LUMINOSITY:
         default:
 	    assert (0);
+	    return kPrivateCGCompositeClear;
     }
 }
 
 static cairo_int_status_t
 _cairo_quartz_surface_set_cairo_operator (cairo_quartz_surface_t *surface, cairo_operator_t op)
 {
     ND((stderr, "%p _cairo_quartz_surface_set_cairo_operator %d\n", surface, op));