diff --git a/lib/CodeGen/CGCXX.cpp b/lib/CodeGen/CGCXX.cpp
index 506f305..cd58b83 100644
--- a/lib/CodeGen/CGCXX.cpp
+++ b/lib/CodeGen/CGCXX.cpp
@@ -317,7 +317,7 @@
   
   // Since the function pointer is 1 plus the virtual table offset, we
   // subtract 1 by using a GEP.
-  VTable = Builder.CreateConstGEP1_64(VTable, -1);
+  VTable = Builder.CreateConstGEP1_64(VTable, (uint64_t)-1);
   
   llvm::Value *VirtualFn = Builder.CreateLoad(VTable, "virtualfn");