diff --git a/lib/carto/tree/call.js b/lib/carto/tree/call.js
index 6ad6de2..a50c79d 100644
--- a/lib/carto/tree/call.js
+++ b/lib/carto/tree/call.js
@@ -23,6 +23,15 @@ tree.Call.prototype = {
     eval: function(env) {
         var args = this.args.map(function(a) { return a.eval(env) });
 
+        for (var i = 0; i < args.length; i++) {
+            if (args[i].is === 'undefined') {
+                return {
+                    is: 'undefined',
+                    value: 'undefined'
+                };
+            }
+        }
+
         if (this.name in tree.functions) { // 1.
             return tree.functions[this.name].apply(tree.functions, args);
         } else { // 2.
diff --git a/lib/carto/tree/operation.js b/lib/carto/tree/operation.js
index 879a419..4c3da44 100644
--- a/lib/carto/tree/operation.js
+++ b/lib/carto/tree/operation.js
@@ -11,6 +11,13 @@ tree.Operation.prototype.eval = function(env) {
         b = this.operands[1].eval(env),
         temp;
 
+    if (a.is === 'undefined' || b.is === 'undefined') {
+        return {
+            is: 'undefined',
+            value: 'undefined'
+        };
+    }
+
     if (a instanceof tree.Dimension && b instanceof tree.Color) {
         if (this.op === '*' || this.op === '+') {
             temp = b, b = a, a = temp;
diff --git a/test/errorhandling/undefined_variable.mss b/test/errorhandling/undefined_variable.mss
index 2c9ea11..15789b0 100644
--- a/test/errorhandling/undefined_variable.mss
+++ b/test/errorhandling/undefined_variable.mss
@@ -1,3 +1,5 @@
 #world[zoom=5] {
   polygon-fill: @something;
+  line-color: @something + #111;
+  marker-fill: darken(@something, 10);
 }