diff --git a/packfile.c b/packfile.c
index 1a714fbde9..7cd45aa4b2 100644
--- a/packfile.c
+++ b/packfile.c
@@ -1454,6 +1454,9 @@ static void *unpack_compressed_entry(struct packed_git *p,
 		return NULL;
 	}
 
+	/* versions of zlib can clobber unconsumed portion of outbuf */
+	buffer[size] = '\0';
+
 	return buffer;
 }