diff --git a/lib/CodeGen/CodeGenPrepare.cpp b/lib/CodeGen/CodeGenPrepare.cpp
index d2559cf53d2..a6d8a68588d 100644
--- a/lib/CodeGen/CodeGenPrepare.cpp
+++ b/lib/CodeGen/CodeGenPrepare.cpp
@@ -2759,7 +2759,7 @@ bool CodeGenPrepare::OptimizeMemoryInst(Instruction *MemoryInst, Value *Addr,
         // the original IR value was tossed in favor of a constant back when
         // the AddrMode was created we need to bail out gracefully if widths
         // do not match instead of extending it.
-        Instruction *I = dyn_cast<Instruction>(Result);
+        Instruction *I = dyn_cast_or_null<Instruction>(Result);
         if (I && (Result != AddrMode.BaseReg))
           I->eraseFromParent();
         return false;
diff --git a/test/CodeGen/X86/codegen-prepare-crash.ll b/test/CodeGen/X86/codegen-prepare-crash.ll
new file mode 100644
index 00000000000..c3288170cc4
--- /dev/null
+++ b/test/CodeGen/X86/codegen-prepare-crash.ll
@@ -0,0 +1,14 @@
+; RUN: llc < %s
+target triple = "x86_64-unknown-linux-gnu"
+
+@g = external global [10 x i32]
+
+define void @f(i32 %u) {
+  %1 = add i32 %u, 4
+  br label %P.Proc8.exit
+
+P.Proc8.exit:
+  %valueindex35.i = getelementptr [10 x i32]* @g, i32 0, i32 %1
+  store i32 %u, i32* %valueindex35.i
+  ret void
+}