diff --git a/glamor/glamor_prepare.c b/glamor/glamor_prepare.c
index 9bfc557..3927a10 100644
--- a/glamor/glamor_prepare.c
+++ b/glamor/glamor_prepare.c
@@ -111,6 +111,7 @@ glamor_prep_pixmap_box(PixmapPtr pixmap, glamor_access_t access, BoxPtr box)
             gl_access = GL_READ_ONLY;
 
         pixmap->devPrivate.ptr = glMapBuffer(GL_PIXEL_PACK_BUFFER, gl_access);
+        assert(pixmap->devPrivate.ptr);
         glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
     }