Index: ironruby/Compiler/AST/Expressions/AstNodeDescriptionExpression.cs
===================================================================
--- ironruby/Compiler/AST/Expressions/AstNodeDescriptionExpression.cs	(revision 76)
+++ ironruby/Compiler/AST/Expressions/AstNodeDescriptionExpression.cs	(working copy)
@@ -35,7 +35,8 @@
         }
 
         internal override MSA.Expression/*!*/ TransformRead(AstGenerator/*!*/ gen) {
-            return Ast.Constant(_expression.NodeName);
+            MSA.Expression name = Ast.Constant(_expression.NodeName);
+            return Ast.Condition(_expression.Defined(gen), name, Ast.Null(name.Type));
         }
     }
 }
Index: ironruby/Compiler/AST/Expressions/Expression.cs
===================================================================
--- ironruby/Compiler/AST/Expressions/Expression.cs	(revision 76)
+++ ironruby/Compiler/AST/Expressions/Expression.cs	(working copy)
@@ -33,6 +33,10 @@
 
         }
 
+        internal virtual MSA.Expression/*!*/ Defined(AstGenerator/*!*/ gen) {
+            return Ast.True();
+        }
+
         internal virtual MSA.Expression/*!*/ TransformRead(AstGenerator/*!*/ gen) {
             throw new NotImplementedException("Expression transformation not implemented: " + GetType().Name);
         }
Index: ironruby/Compiler/AST/LeftValues/ConstantVariable.cs
===================================================================
--- ironruby/Compiler/AST/LeftValues/ConstantVariable.cs	(revision 76)
+++ ironruby/Compiler/AST/LeftValues/ConstantVariable.cs	(working copy)
@@ -94,6 +94,27 @@
             }
         }
 
+
+        internal override MSA.Expression/*!*/ Defined(AstGenerator/*!*/ gen) {
+            MSA.Expression transformedName = TransformSymbolName(gen);
+            MSA.Expression transformedQualifier;
+
+            switch (TransformQualifier(gen, out transformedQualifier))
+            {
+                case StaticScopeKind.Global:
+                    return AstFactory.OpCall("GlobalConstantDefined", Ast.CodeContext(), transformedName);
+
+                case StaticScopeKind.EnclosingModule:
+                    return AstFactory.OpCall("ImplicitConstantDefined", Ast.CodeContext(), transformedName);
+
+                case StaticScopeKind.Explicit:
+                    return AstFactory.OpCall("ExplicitConstantDefined", transformedQualifier, Ast.CodeContext(), transformedName);
+            }
+
+            throw Assert.Unreachable;
+        }
+
+
         internal override MSA.Expression/*!*/ TransformRead(AstGenerator/*!*/ gen) {
             MSA.Expression transformedName = TransformSymbolName(gen);
             MSA.Expression transformedQualifier;
Index: ironruby/Runtime/RubyOps.cs
===================================================================
--- ironruby/Runtime/RubyOps.cs	(revision 76)
+++ ironruby/Runtime/RubyOps.cs	(working copy)
@@ -277,6 +277,30 @@
         #region Constants
 
         // emitted:
+        public static bool GlobalConstantDefined(CodeContext/*!*/ context, SymbolId name)
+        {
+            RubyExecutionContext ec = RubyUtils.GetExecutionContext(context);
+            object result;
+            return ec.ObjectClass.TryResolveConstant(name, out result);
+        }
+
+        // emitted:
+        public static bool ImplicitConstantDefined(CodeContext/*!*/ context, SymbolId name)
+        {
+            RubyExecutionContext ec = RubyUtils.GetExecutionContext(context);
+            object result;
+            return ec.TryResolveConstant(context, name, out result);
+        }
+
+        // emitted:
+        public static bool ExplicitConstantDefined(object target, CodeContext/*!*/ context, SymbolId name)
+        {
+            object result;
+            return RubyUtils.GetModuleFromObject(context, target).TryResolveConstant(name, out result);
+        }
+
+
+        // emitted:
         public static object GetGlobalConstant(CodeContext/*!*/ context, SymbolId name) {
             RubyExecutionContext ec = RubyUtils.GetExecutionContext(context);
             return RubyUtils.GetConstant(context, ec.ObjectClass, name);