diff --git a/lib/Analysis/ConstantFolding.cpp b/lib/Analysis/ConstantFolding.cpp
index 7a0a4e1e824..783c32e6669 100644
--- a/lib/Analysis/ConstantFolding.cpp
+++ b/lib/Analysis/ConstantFolding.cpp
@@ -681,6 +681,7 @@ static Constant *SymbolicallyEvaluateGEP(ArrayRef<Constant *> Ops,
   // This makes it easy to determine if the getelementptr is "inbounds".
   // Also, this helps GlobalOpt do SROA on GlobalVariables.
   Type *Ty = Ptr->getType();
+  assert(Ty->isPointerTy() && "Forming regular GEP of non-pointer type");
   SmallVector<Constant*, 32> NewIdxs;
   do {
     if (SequentialType *ATy = dyn_cast<SequentialType>(Ty)) {
@@ -711,10 +712,17 @@ static Constant *SymbolicallyEvaluateGEP(ArrayRef<Constant *> Ops,
       }
       Ty = ATy->getElementType();
     } else if (StructType *STy = dyn_cast<StructType>(Ty)) {
-      // Determine which field of the struct the offset points into. The
-      // getZExtValue is at least as safe as the StructLayout API because we
-      // know the offset is within the struct at this point.
+      // If we end up with an offset that isn't valid for this struct type, we
+      // can't re-form this GEP in a regular form, so bail out. The pointer
+      // operand likely went through casts that are necessary to make the GEP
+      // sensible.
       const StructLayout &SL = *TD->getStructLayout(STy);
+      if (Offset.uge(SL.getSizeInBytes()))
+        break;
+
+      // Determine which field of the struct the offset points into. The
+      // getZExtValue is fine as we've already ensured that the offset is
+      // within the range representable by the StructLayout API.
       unsigned ElIdx = SL.getElementContainingOffset(Offset.getZExtValue());
       NewIdxs.push_back(ConstantInt::get(Type::getInt32Ty(Ty->getContext()),
                                          ElIdx));
diff --git a/test/Transforms/GlobalOpt/constantfold-initializers.ll b/test/Transforms/GlobalOpt/constantfold-initializers.ll
index af8fa324db8..ce6e2c46d62 100644
--- a/test/Transforms/GlobalOpt/constantfold-initializers.ll
+++ b/test/Transforms/GlobalOpt/constantfold-initializers.ll
@@ -12,6 +12,11 @@ target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f3
 @xs = global [2 x i32] zeroinitializer, align 4
 ; CHECK: @xs = global [2 x i32] [i32 1, i32 1]
 
+; PR12642
+%PR12642.struct = type { i8 }
+@PR12642.s = global <{}> zeroinitializer, align 1
+@PR12642.p = constant %PR12642.struct* bitcast (i8* getelementptr (i8* bitcast (<{}>* @PR12642.s to i8*), i64 1) to %PR12642.struct*), align 8
+
 define internal void @test1() {
 entry:
   store i32 1, i32* getelementptr inbounds ([2 x i32]* @xs, i64 0, i64 0)