diff --git a/Interpreter.cs b/Interpreter.cs
index e1bad2e..fd90310 100644
--- a/Interpreter.cs
+++ b/Interpreter.cs
@@ -532,8 +532,8 @@ public class Interpreter : AST.IExprVisitor<Env, object>
 
 			}
 			extLabels.Add(label);
-			if (extension.Value == null) throw new NotImplementedException();
-			rec = rec.Extend(label, evaluate(env, extension.Value));
+			var extensionValue = extension.Value == null ? env[extension.Name] : evaluate(env, extension.Value);
+			rec = rec.Extend(label, extensionValue);
 		}
 
 		return rec;