diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index cbcc85944a75de4e425d6cf3754ce17cce518728..b41cdc65202f6dc6edbc00765a58b0c7dea9785b 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -900,6 +900,10 @@ Instruction *InstCombiner::transformSExtICmp(ICmpInst *ICI, Instruction &CI) {
   Value *Op0 = ICI->getOperand(0), *Op1 = ICI->getOperand(1);
   ICmpInst::Predicate Pred = ICI->getPredicate();
 
+  // Don't bother if Op1 isn't of vector or integer type.
+  if (!Op1->getType()->isIntOrIntVectorTy())
+    return nullptr;
+
   if (Constant *Op1C = dyn_cast<Constant>(Op1)) {
     // (x <s  0) ? -1 : 0 -> ashr x, 31        -> all ones if negative
     // (x >s -1) ? -1 : 0 -> not (ashr x, 31)  -> all ones if positive
diff --git a/llvm/test/Transforms/InstCombine/cast.ll b/llvm/test/Transforms/InstCombine/cast.ll
index 7d6a855d58e3366b9e69a203f9f59dbc03134097..578b16d981135bd5a3b8f0530a6636de9f26a058 100644
--- a/llvm/test/Transforms/InstCombine/cast.ll
+++ b/llvm/test/Transforms/InstCombine/cast.ll
@@ -1084,3 +1084,12 @@ define float @overflow_sitofp() {
 ; CHECK-NEXT: ret float undef 
 }
 
+define i32 @PR21388(i32* %v) {
+  %icmp = icmp slt i32* %v, null
+  %sext = sext i1 %icmp to i32
+  ret i32 %sext
+; CHECK-LABEL: @PR21388(
+; CHECK-NEXT: %[[icmp:.*]] = icmp slt i32* %v, null
+; CHECK-NEXT: %[[sext:.*]] = sext i1 %[[icmp]] to i32
+; CHECK-NEXT: ret i32 %[[sext]]
+}