diff --git a/llvm/lib/Target/X86/X86RetpolineThunks.cpp b/llvm/lib/Target/X86/X86RetpolineThunks.cpp
index d03826bbe992..f37c3fbc3994 100644
--- a/llvm/lib/Target/X86/X86RetpolineThunks.cpp
+++ b/llvm/lib/Target/X86/X86RetpolineThunks.cpp
@@ -214,6 +214,15 @@ void X86RetpolineThunks::createThunkFunction(Module &M, StringRef Name) {
   IRBuilder<> Builder(Entry);
 
   Builder.CreateRetVoid();
+
+  // MachineFunctions/MachineBasicBlocks aren't created automatically for the
+  // IR-level constructs we already made. Create them and insert them into the
+  // module.
+  MachineFunction &MF = MMI->getOrCreateMachineFunction(*F);
+  MachineBasicBlock *EntryMBB = MF.CreateMachineBasicBlock(Entry);
+
+  // Insert EntryMBB into MF. It's not in the module until we do this.
+  MF.insert(MF.end(), EntryMBB);
 }
 
 void X86RetpolineThunks::insertRegReturnAddrClobber(MachineBasicBlock &MBB,