diff --git a/lib/Transforms/Utils/ValueMapper.cpp b/lib/Transforms/Utils/ValueMapper.cpp
index 414d4ea..c8d1338 100644
--- a/lib/Transforms/Utils/ValueMapper.cpp
+++ b/lib/Transforms/Utils/ValueMapper.cpp
@@ -42,9 +42,16 @@
 
   // Global values do not need to be seeded into the VM if they
   // are using the identity mapping.
-  if (isa<GlobalValue>(V))
+  if (isa<GlobalValue>(V)) {
+    if (Flags & RF_NullMapMissingGlobalValues) {
+      assert(!(Flags & RF_IgnoreMissingEntries) &&
+             "Illegal to specify both RF_NullMapMissingGlobalValues and "
+             "RF_IgnoreMissingEntries");
+      return nullptr;
+    }
     return VM[V] = const_cast<Value*>(V);
-  
+  }
+
   if (const InlineAsm *IA = dyn_cast<InlineAsm>(V)) {
     // Inline asm may need *type* remapping.
     FunctionType *NewTy = IA->getFunctionType();
@@ -74,7 +81,8 @@
     // correct.  For now, just match behaviour from before the metadata/value
     // split.
     //
-    //    assert(MappedMD && "Referenced metadata value not in value map");
+    //    assert((MappedMD || (Flags & RF_NullMapMissingGlobalValues)) &&
+    //           "Referenced metadata value not in value map");
     return VM[V] = MetadataAsValue::get(V->getContext(), MappedMD);
   }
 
@@ -184,7 +192,8 @@
   // correct.  For now, just match behaviour from before the metadata/value
   // split.
   //
-  //    llvm_unreachable("Referenced metadata not in value map!");
+  //    assert((Flags & RF_NullMapMissingGlobalValues) &&
+  //           "Referenced metadata not in value map!");
   return nullptr;
 }
 
@@ -305,7 +314,8 @@
     // correct.  For now, just match behaviour from before the metadata/value
     // split.
     //
-    //    assert(MappedV && "Referenced metadata not in value map!");
+    //    assert((MappedV || (Flags & RF_NullMapMissingGlobalValues)) &&
+    //           "Referenced metadata not in value map!");
     if (MappedV)
       return mapToMetadata(VM, MD, ValueAsMetadata::get(MappedV));
     return nullptr;