Report error when variable or function is not found

VAR.ExpressionEvaluator/ExpressionFunctionNode.cs

@@ -17,11 +17,12 @@ namespace VAR.ExpressionEvaluator
         public object Eval(IEvaluationContext evaluationContext)
         {
             object[] paramValues = _paramNodes.Select(p => p.Eval(evaluationContext)).ToArray();
-
             Func<object[], object> func = evaluationContext.GetFunction(_name);
-
+            if (func == null)
+            {
+                throw new Exception(string.Format("Function {0} not found", _name));
+            }
             object result = func(paramValues);
-
             return result;
         }
     }

VAR.ExpressionEvaluator/ExpressionVariableNode.cs

@@ -1,4 +1,6 @@
-namespace VAR.ExpressionEvaluator
+using System;
+
+namespace VAR.ExpressionEvaluator
 {
     public class ExpressionVariableNode : IExpressionNode
     {
@@ -11,7 +13,12 @@
 
         public object Eval(IEvaluationContext evaluationContext)
         {
-            return evaluationContext.GetVariable(_name);
+            object value = evaluationContext.GetVariable(_name);
+            if (value == null)
+            {
+                throw new Exception(string.Format("Variable {0} not found", _name));
+            }
+            return value;
         }
     }
 }