diff --git a/include/llvm/CodeGen/LiveIntervalAnalysis.h b/include/llvm/CodeGen/LiveIntervalAnalysis.h
index c0de49e52ab..46323681577 100644
--- a/include/llvm/CodeGen/LiveIntervalAnalysis.h
+++ b/include/llvm/CodeGen/LiveIntervalAnalysis.h
@@ -218,7 +218,7 @@ namespace llvm {
       Indexes->insertMBBInMaps(MBB);
       assert(unsigned(MBB->getNumber()) == RegMaskBlocks.size() &&
              "Blocks must be added in order.");
-      RegMaskBlocks.push_back(std::make_pair(MBB->getNumber(), 0));
+      RegMaskBlocks.push_back(std::make_pair(RegMaskSlots.size(), 0));
     }
 
     SlotIndex InsertMachineInstrInMaps(MachineInstr *MI) {