diff --git a/llvm/lib/Transforms/IPO/IROutliner.cpp b/llvm/lib/Transforms/IPO/IROutliner.cpp
--- a/llvm/lib/Transforms/IPO/IROutliner.cpp
+++ b/llvm/lib/Transforms/IPO/IROutliner.cpp
@@ -170,6 +170,15 @@
     SortedKeys.push_back(VtoBB.first);
 
   stable_sort(SortedKeys, [](const Value *LHS, const Value *RHS) {
+    // We may be dealing with a return void here in which case Value* will be
+    // nullptr. Assume that void is < any ConstantInt.
+    if ((LHS == nullptr) && (RHS == nullptr))
+      return false;
+    if (LHS == nullptr)
+      return true;
+    if (RHS == nullptr)
+      return false;
+
     const ConstantInt *LHSC = dyn_cast<ConstantInt>(LHS);
     const ConstantInt *RHSC = dyn_cast<ConstantInt>(RHS);
     assert(RHSC && "Not a constant integer in return value?");