Index: llvm/trunk/include/llvm/ADT/Twine.h
===================================================================
--- llvm/trunk/include/llvm/ADT/Twine.h
+++ llvm/trunk/include/llvm/ADT/Twine.h
@@ -274,6 +274,9 @@
 
       assert(isValid() && "Invalid twine!");
     }
+    /// Delete the implicit conversion from nullptr as Twine(const char *)
+    /// cannot take nullptr.
+    /*implicit*/ Twine(std::nullptr_t) = delete;
 
     /// Construct from an std::string.
     /*implicit*/ Twine(const std::string &Str) : LHSKind(StdStringKind) {
Index: llvm/trunk/include/llvm/IR/IRBuilder.h
===================================================================
--- llvm/trunk/include/llvm/IR/IRBuilder.h
+++ llvm/trunk/include/llvm/IR/IRBuilder.h
@@ -1004,7 +1004,7 @@
   }
 
   Value *foldConstant(Instruction::BinaryOps Opc, Value *L,
-                      Value *R, const Twine &Name = nullptr) const {
+                      Value *R, const Twine &Name) const {
     auto *LC = dyn_cast<Constant>(L);
     auto *RC = dyn_cast<Constant>(R);
     return (LC && RC) ? Insert(Folder.CreateBinOp(Opc, LC, RC), Name) : nullptr;