Index: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -5917,7 +5917,10 @@
     // to detect it as a final shuffled/identity match.
     if (auto *VU = dyn_cast_or_null<InsertElementInst>(EU.User)) {
       if (auto *FTy = dyn_cast<FixedVectorType>(VU->getType())) {
-        unsigned InsertIdx = *getInsertIndex(VU);
+        Optional<unsigned> Idx = getInsertIndex(VU);
+        if (!Idx)
+          continue;
+        unsigned InsertIdx = *Idx;
         auto *It = find_if(FirstUsers, [VU](Value *V) {
           return areTwoInsertFromSameBuildVector(VU,
                                                  cast<InsertElementInst>(V));
Index: llvm/test/Transforms/SLPVectorizer/slp-variable-insertelement.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/SLPVectorizer/slp-variable-insertelement.ll
@@ -0,0 +1,31 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -slp-vectorizer -slp-max-vf=2 -slp-min-reg-size=32 -S < %s | FileCheck %s
+
+; It is possible to compute the tree cost for an insertelement that does not
+; have a constant index when the index is a PHI. Check if getInsertIndex
+; returns None.
+
+define void @test() local_unnamed_addr {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
+; CHECK:       for.cond.cleanup:
+; CHECK-NEXT:    unreachable
+; CHECK:       for.body:
+; CHECK-NEXT:    [[I:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ poison, [[FOR_BODY]] ]
+; CHECK-NEXT:    [[J:%.*]] = phi i32 [ poison, [[ENTRY]] ], [ 0, [[FOR_BODY]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = insertelement <4 x i32> poison, i32 poison, i32 [[I]]
+; CHECK-NEXT:    br i1 poison, label [[FOR_COND_CLEANUP:%.*]], label [[FOR_BODY]]
+;
+entry:
+  br label %for.body
+
+for.cond.cleanup:
+  unreachable
+
+for.body:
+  %i = phi i32 [ 0, %entry ], [ poison, %for.body ]
+  %j = phi i32 [ poison, %entry ], [ 0, %for.body ]
+  %0 = insertelement <4 x i32> poison, i32 poison, i32 %i
+  br i1 poison, label %for.cond.cleanup, label %for.body
+}