diff --git a/lib/VMCore/Instructions.cpp b/lib/VMCore/Instructions.cpp
index 754d992e6b0..95348d91f23 100644
--- a/lib/VMCore/Instructions.cpp
+++ b/lib/VMCore/Instructions.cpp
@@ -2462,6 +2462,7 @@ CastInst::getCastOpcode(
     if (const VectorType *SrcPTy = dyn_cast<VectorType>(SrcTy)) {
       assert(SrcPTy->getBitWidth()==64 &&
              "Casting vector of wrong width to X86_MMX");
+      SrcPTy = NULL;      // inhibit compiler warning
       return BitCast;                               // 64-bit vector to MMX
     } else {
       assert(!"Illegal cast to X86_MMX");