diff --git a/src/cairo-xlib-surface-shm.c b/src/cairo-xlib-surface-shm.c
index 44e6284b1..0ca4305c9 100644
--- a/src/cairo-xlib-surface-shm.c
+++ b/src/cairo-xlib-surface-shm.c
@@ -55,7 +55,7 @@ _cairo_xlib_surface_get_shm (cairo_xlib_surface_t *surface,
 cairo_int_status_t
 _cairo_xlib_surface_put_shm (cairo_xlib_surface_t *surface)
 {
-    ASSERT_NOT_REACHED;
+    assert (!surface->fallback);
     return CAIRO_INT_STATUS_SUCCESS;
 }