diff --git a/lib/validation.js b/lib/validation.js
index e1e1b96..175fb49 100644
--- a/lib/validation.js
+++ b/lib/validation.js
@@ -176,11 +176,23 @@ module.exports.process = function (validationModel, req, options) {
                     keys.splice(index, 1);
                 }
             });
-            if (options.forbidUndefinedVariables === true) {
-                keys.forEach(function(key) {
-                    addError(_createError(realScope, key, { name: "undefinedVariable" }));
-                });
-            }
+    	    if (options.forbidUndefinedVariables === true) {
+    	        keys.forEach(function(key) {
+                    var invalid = true;
+
+                    // check if key exist in some other scope in the validationModel
+                    _.each(validationModel, function (validationModelScope) {
+                        if (key in validationModelScope) {
+                            invalid = false;
+                        }
+                    });
+
+                    // flag key as undefined if it didn't show up in another scope
+                    if (invalid) {
+                        addError(_createError(realScope, key, { name: "undefinedVariable" }));
+                    }
+    	        });
+    	    }
         }
     });
     return errors;