diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 3e45252..083f9c4 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -1111,8 +1111,9 @@ unsigned int btrfs_compress_str2level(const char *str)
 	if (strncmp(str, "zlib", 4) != 0)
 		return 0;
 
-	if ('1' <= str[4] && str[4] <= '9' )
-		return str[4] - '0';
+	/* Accepted form: zlib:1 up to zlib:9 and nothing left after the number */
+	if (str[4] == ':' && '1' <= str[5] && str[5] <= '9' && str[6] == 0)
+		return str[5] - '0';
 
 	return 0;
 }