diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -661,6 +661,10 @@
 
 // Check V's users to see if it is involved in a reduction in L.
 static PHINode *findInnerReductionPhi(Loop *L, Value *V) {
+  // Reduction variables cannot be constants.
+  if (isa<Constant>(V))
+    return nullptr;
+
   for (Value *User : V->users()) {
     if (PHINode *PHI = dyn_cast<PHINode>(User)) {
       if (PHI->getNumIncomingValues() == 1)
diff --git a/llvm/test/Transforms/LoopInterchange/reductions-across-inner-and-outer-loop.ll b/llvm/test/Transforms/LoopInterchange/reductions-across-inner-and-outer-loop.ll
--- a/llvm/test/Transforms/LoopInterchange/reductions-across-inner-and-outer-loop.ll
+++ b/llvm/test/Transforms/LoopInterchange/reductions-across-inner-and-outer-loop.ll
@@ -150,3 +150,42 @@
   %sum.inc.lcssa2 = phi i64 [ %sum.inc.lcssa, %for1.inc ]
   ret i64 %sum.inc.lcssa2
 }
+
+; Check that we do not interchange or crash if the PHI in the outer loop gets a
+; constant from the inner loop.
+; REMARKS: --- !Missed
+; REMARKS-NEXT: Pass:            loop-interchange
+; REMARKS-NEXT: Name:            UnsupportedPHIOuter
+; REMARKS-NEXT: Function:        test_constant_inner_loop_res
+
+define i64 @test_constant_inner_loop_res([100 x [100 x i64]]* %Arr) {
+entry:
+  br label %for1.header
+
+for1.header:                                         ; preds = %for1.inc, %entry
+  %indvars.iv23 = phi i64 [ 0, %entry ], [ %indvars.iv.next24, %for1.inc ]
+  %sum.outer = phi i64 [ 0, %entry ], [ %sum.inc.amend, %for1.inc ]
+  br label %for2
+
+for2:                                        ; preds = %for2, %for1.header
+  %indvars.iv = phi i64 [ 0, %for1.header ], [ %indvars.iv.next.3, %for2 ]
+  %sum.inner = phi i64 [ %sum.outer, %for1.header ], [ %sum.inc, %for2 ]
+  %arrayidx = getelementptr inbounds [100 x [100 x i64]], [100 x [100 x i64]]* %Arr, i64 0, i64 %indvars.iv, i64 %indvars.iv23
+  %lv = load i64, i64* %arrayidx, align 4
+  %sum.inc = add i64 %sum.inner, %lv
+  %indvars.iv.next.3 = add nuw nsw i64 %indvars.iv, 1
+  %exit1 = icmp eq i64 %indvars.iv.next.3, 100
+  br i1 %exit1, label %for1.inc, label %for2
+
+for1.inc:                                ; preds = %for2
+  %sum.inc.lcssa = phi i64 [ %sum.inc, %for2 ]
+  %const.lcssa = phi i64 [ 0, %for2 ]
+  %sum.inc.amend = add i64 %const.lcssa, %sum.inc.lcssa
+  %indvars.iv.next24 = add nuw nsw i64 %indvars.iv23, 1
+  %exit2 = icmp eq i64 %indvars.iv.next24, 100
+  br i1 %exit2, label %for1.loopexit, label %for1.header
+
+for1.loopexit:                                 ; preds = %for1.inc
+  %il.res.lcssa2 = phi i64 [ %sum.inc.amend, %for1.inc ]
+  ret i64 %il.res.lcssa2
+}