diff --git a/lib/generate.js b/lib/generate.js
index <HASH>..<HASH> 100644
--- a/lib/generate.js
+++ b/lib/generate.js
@@ -83,10 +83,15 @@ function serializeRule(rule, builtinPostprocessors) {
 }
 
 var generate = function (parser, exportName) {
-    return (generate[parser.config.preprocessor] ?
-      generate[parser.config.preprocessor] :
-      generate["_default"]
-    )(parser, exportName);
+    if(!parser.config.preprocessor) {
+      parser.config.preprocessor = "_default";
+    }
+
+    if(!generate[parser.config.preprocessor]) {
+      throw new Error("No such preprocessor: " + parser.config.preprocessor)
+    }
+
+    return generate[parser.config.preprocessor](parser, exportName);
 };
 
 generate.js = generate._default = generate.javascript = function (parser, exportName) {
Throw an Error when the preprocessor isn't found.