diff --git a/src/hb-open-type-private.hh b/src/hb-open-type-private.hh
index 5616f04de..477d9e28b 100644
--- a/src/hb-open-type-private.hh
+++ b/src/hb-open-type-private.hh
@@ -194,6 +194,7 @@ struct hb_sanitize_context_t
   {
     this->start = hb_blob_get_data (this->blob, NULL);
     this->end = this->start + hb_blob_get_length (this->blob);
+    assert (this->start <= this->end); /* Must not overflow. */
     this->edit_count = 0;
     this->debug_depth = 0;