diff --git a/src/runtime/base/hphp_system.cpp b/src/runtime/base/hphp_system.cpp
index 12fc3f7721b08..0d15ad6dbf893 100644
--- a/src/runtime/base/hphp_system.cpp
+++ b/src/runtime/base/hphp_system.cpp
@@ -92,17 +92,19 @@ void Globals::initialize() {
   }
 }
 
-CVarRef Globals::declareConstant(CStrRef name, Variant &constant,
-                                 CVarRef value) {
+bool Globals::declareConstant(CStrRef name, Variant &constant,
+                              CVarRef value) {
   if (!value.isAllowedAsConstantValue()) {
     raise_warning("Constants may only evaluate to scalar values");
-    return false_varNR;
+    return false;
   }
   if (!m_dynamicConstants.exists(name)) {
     m_dynamicConstants.set(name, value);
     constant = value;
+    return true;
   }
-  return value;
+  raise_warning("Constant %s already defined", name.data());
+  return false;
 }
 
 void Globals::declareFunction(const char *name) {
diff --git a/src/runtime/base/hphp_system.h b/src/runtime/base/hphp_system.h
index bfa380e27a79d..7ab6e07d100dc 100644
--- a/src/runtime/base/hphp_system.h
+++ b/src/runtime/base/hphp_system.h
@@ -53,7 +53,7 @@ class Globals : public LVariableTable {
 public:
   Globals() : FVF(__autoload)(false) {}
   void initialize();
-  CVarRef declareConstant(CStrRef name, Variant &constant, CVarRef value);
+  bool declareConstant(CStrRef name, Variant &constant, CVarRef value);
   void declareFunction(const char *name);
   void declareFunctionLit(CStrRef name);
   bool defined(CStrRef name);
diff --git a/src/test/test_code_run.cpp b/src/test/test_code_run.cpp
index ab03ac7a9ef4c..a237825882176 100644
--- a/src/test/test_code_run.cpp
+++ b/src/test/test_code_run.cpp
@@ -14629,6 +14629,10 @@ bool TestCodeRun::TestConstant() {
        "var_dump(BLAH);"
        "define('FOO', array(1,2,3));");
 
+  MVCR("<?php "
+       "var_dump(define('foo', false));"
+       "var_dump(define('foo', true));");
+
   return true;
 }