diff --git a/src/cairo-mempool.c b/src/cairo-mempool.c
index 96e4a62b2..751ede320 100644
--- a/src/cairo-mempool.c
+++ b/src/cairo-mempool.c
@@ -157,7 +157,8 @@ get_buddy (cairo_mempool_t *pool, size_t offset, int bits)
 {
     struct _cairo_memblock *block;
 
-    assert (offset + (1 << bits) <= pool->num_blocks);
+    if (offset + (1 << bits) >= pool->num_blocks)
+	return NULL; /* invalid */
 
     if (BITTEST (pool, offset + (1 << bits) - 1))
 	return NULL; /* buddy is allocated */