diff --git a/Upsilon/Evaluator/EvaluationScope.cs b/Upsilon/Evaluator/EvaluationScope.cs
index 7cd6504..51b1b97 100644
--- a/Upsilon/Evaluator/EvaluationScope.cs
+++ b/Upsilon/Evaluator/EvaluationScope.cs
@@ -40,17 +40,20 @@ namespace Upsilon.Evaluator
 
         public void AssignToNearest(VariableSymbol symbol, ScriptType value)
         {
-            if (Variables.ContainsKey(symbol.Name))
+            lock (Variables)
             {
-                Variables[symbol.Name] = value;
-            }
-            else if (ParentScope != null)
-            {
-                ParentScope.AssignToNearest(symbol, value);
-            }
-            else
-            {
-                Variables.Add(symbol.Name, value);
+                if (Variables.ContainsKey(symbol.Name))
+                {
+                    Variables[symbol.Name] = value;
+                }
+                else if (ParentScope != null)
+                {
+                    ParentScope.AssignToNearest(symbol, value);
+                }
+                else
+                {
+                    Variables.Add(symbol.Name, value);
+                }
             }
         }