diff --git a/lib/preprocess.js b/lib/preprocess.js
index f7b1ce9..600e5f8 100644
--- a/lib/preprocess.js
+++ b/lib/preprocess.js
@@ -50,7 +50,9 @@ function preprocess(src,context,type) {
   src = src.toString();
   context = context || process.env;
 
-  type = type || 'html';
+  if (typeof delim[type] === 'undefined'){
+    type = 'html';
+  }
 
   var rv = src;