diff --git a/src/hb-buffer-serialize.cc b/src/hb-buffer-serialize.cc
index 63a0f34..85696c5 100644
--- a/src/hb-buffer-serialize.cc
+++ b/src/hb-buffer-serialize.cc
@@ -311,6 +311,8 @@
   if (!buf_consumed)
     buf_consumed = &sconsumed;
   *buf_consumed = 0;
+  if (buf_size)
+    *buf = '\0';
 
   assert ((!buffer->len && buffer->content_type == HB_BUFFER_CONTENT_TYPE_INVALID) ||
 	  buffer->content_type == HB_BUFFER_CONTENT_TYPE_GLYPHS);