diff --git a/src/mesa/state_tracker/st_cb_bitmap.c b/src/mesa/state_tracker/st_cb_bitmap.c
index 63dbdb2..4b52522 100644
--- a/src/mesa/state_tracker/st_cb_bitmap.c
+++ b/src/mesa/state_tracker/st_cb_bitmap.c
@@ -688,6 +688,12 @@ accum_bitmap(struct st_context *st,
        height > BITMAP_CACHE_HEIGHT)
       return GL_FALSE; /* too big to cache */
 
+   /* PBO source... */
+   bitmap = _mesa_map_pbo_source(st->ctx, unpack, bitmap);
+   if (!bitmap) {
+      return GL_FALSE;
+   }
+
    if (!cache->empty) {
       px = x - cache->xpos;  /* pos in buffer */
       py = y - cache->ypos;