diff --git a/llvm/include/llvm/Transforms/Utils/FunctionComparator.h b/llvm/include/llvm/Transforms/Utils/FunctionComparator.h
index e0c79a1027e6..7698a068717a 100644
--- a/llvm/include/llvm/Transforms/Utils/FunctionComparator.h
+++ b/llvm/include/llvm/Transforms/Utils/FunctionComparator.h
@@ -78,6 +78,10 @@ public:
     return MapIter->second;
   }
 
+  void erase(GlobalValue *Global) {
+    GlobalNumbers.erase(Global);
+  }
+
   void clear() {
     GlobalNumbers.clear();
   }
diff --git a/llvm/lib/Transforms/IPO/MergeFunctions.cpp b/llvm/lib/Transforms/IPO/MergeFunctions.cpp
index bffbb8f060de..512de8f5c7e5 100644
--- a/llvm/lib/Transforms/IPO/MergeFunctions.cpp
+++ b/llvm/lib/Transforms/IPO/MergeFunctions.cpp
@@ -629,6 +629,9 @@ void MergeFunctions::filterInstsUnrelatedToPDI(
 void MergeFunctions::writeThunk(Function *F, Function *G) {
   if (!G->isInterposable() && !MergeFunctionsPDI) {
     if (G->hasGlobalUnnamedAddr()) {
+      // G might have been a key in our GlobalNumberState, and it's illegal
+      // to replace a key in ValueMap<GlobalValue *> with a non-global.
+      GlobalNumbers.erase(G);
       // If G's address is not significant, replace it entirely.
       Constant *BitcastF = ConstantExpr::getBitCast(F, G->getType());
       G->replaceAllUsesWith(BitcastF);