diff --git a/lib/shared-method.js b/lib/shared-method.js
index c7a62ca..097f74a 100644
--- a/lib/shared-method.js
+++ b/lib/shared-method.js
@@ -415,12 +415,16 @@ SharedMethod.getType = function(val, targetType) {
  */
 
 SharedMethod.convertArg = function(accept, raw) {
-  if (accept.http && (accept.http.source === 'req' ||
+  var isComputedArgument = accept.http && (
+    typeof accept.http === 'function' ||
+    accept.http.source === 'req' ||
     accept.http.source === 'res' ||
-    accept.http.source === 'context'
-    )) {
+    accept.http.source === 'context');
+
+  if (isComputedArgument) {
     return raw;
   }
+
   if (raw === null || typeof raw !== 'object') {
     return raw;
   }