diff --git a/src/cairo-xlib-display.c b/src/cairo-xlib-display.c
index 8b1fa0a3e..d10ed1e0b 100644
--- a/src/cairo-xlib-display.c
+++ b/src/cairo-xlib-display.c
@@ -125,9 +125,6 @@ _cairo_xlib_display_discard_screens (cairo_xlib_display_t *display)
 cairo_xlib_display_t *
 _cairo_xlib_display_reference (cairo_xlib_display_t *display)
 {
-    if (display == NULL)
-	return NULL;
-
     assert (CAIRO_REFERENCE_COUNT_HAS_REFERENCE (&display->ref_count));
 
     _cairo_reference_count_inc (&display->ref_count);
@@ -138,9 +135,6 @@ _cairo_xlib_display_reference (cairo_xlib_display_t *display)
 void
 _cairo_xlib_display_destroy (cairo_xlib_display_t *display)
 {
-    if (display == NULL)
-	return;
-
     assert (CAIRO_REFERENCE_COUNT_HAS_REFERENCE (&display->ref_count));
 
     if (! _cairo_reference_count_dec_and_test (&display->ref_count))
@@ -213,6 +207,7 @@ _cairo_xlib_close_display (Display *dpy, XExtCodes *codes)
     }
     CAIRO_MUTEX_UNLOCK (_cairo_xlib_display_mutex);
 
+    assert (display != NULL);
     _cairo_xlib_display_destroy (display);
 
     /* Return value in accordance with requirements of
diff --git a/src/cairo-xlib-screen.c b/src/cairo-xlib-screen.c
index f8d593608..b759a85e3 100644
--- a/src/cairo-xlib-screen.c
+++ b/src/cairo-xlib-screen.c
@@ -244,9 +244,6 @@ _cairo_xlib_init_screen_font_options (Display *dpy, cairo_xlib_screen_info_t *in
 cairo_xlib_screen_info_t *
 _cairo_xlib_screen_info_reference (cairo_xlib_screen_info_t *info)
 {
-    if (info == NULL)
-	return NULL;
-
     assert (CAIRO_REFERENCE_COUNT_HAS_REFERENCE (&info->ref_count));
 
     _cairo_reference_count_inc (&info->ref_count);
@@ -273,9 +270,6 @@ _cairo_xlib_screen_info_destroy (cairo_xlib_screen_info_t *info)
     cairo_xlib_screen_info_t **prev;
     cairo_xlib_screen_info_t *list;
 
-    if (info == NULL)
-	return;
-
     assert (CAIRO_REFERENCE_COUNT_HAS_REFERENCE (&info->ref_count));
 
     if (! _cairo_reference_count_dec_and_test (&info->ref_count))