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
@@ -169,14 +169,21 @@
   for (auto &VtoBB : Map)
     SortedKeys.push_back(VtoBB.first);
 
-  stable_sort(SortedKeys, [](const Value *LHS, const Value *RHS) {
-    const ConstantInt *LHSC = dyn_cast<ConstantInt>(LHS);
-    const ConstantInt *RHSC = dyn_cast<ConstantInt>(RHS);
-    assert(RHSC && "Not a constant integer in return value?");
-    assert(LHSC && "Not a constant integer in return value?");
-
-    return LHSC->getLimitedValue() < RHSC->getLimitedValue();
-  });
+  // Here we expect to have either 1 value that is void (nullptr) or multiple
+  // values that are all constant integers.
+  if (SortedKeys.size() == 1)
+    assert(!SortedKeys[0] && "Expected a single void value.");
+  else {
+    stable_sort(SortedKeys, [](const Value *LHS, const Value *RHS) {
+      assert(LHS && RHS && "Expected non void values.");
+      const ConstantInt *LHSC = dyn_cast<ConstantInt>(LHS);
+      const ConstantInt *RHSC = dyn_cast<ConstantInt>(RHS);
+      assert(RHSC && "Not a constant integer in return value?");
+      assert(LHSC && "Not a constant integer in return value?");
+
+      return LHSC->getLimitedValue() < RHSC->getLimitedValue();
+    });
+  }
 }
 
 Value *OutlinableRegion::findCorrespondingValueIn(const OutlinableRegion &Other,