diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index bdf3f44742bdd..9a69ddbad5d58 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -1640,9 +1640,12 @@ class BoUpSLP {
         if (Mask.size() != VL.size() && VL.size() == Scalars.size())
           return std::equal(VL.begin(), VL.end(), Scalars.begin());
         return VL.size() == Mask.size() &&
-               std::equal(
-                   VL.begin(), VL.end(), Mask.begin(),
-                   [Scalars](Value *V, int Idx) { return V == Scalars[Idx]; });
+               std::equal(VL.begin(), VL.end(), Mask.begin(),
+                          [Scalars](Value *V, int Idx) {
+                            return (isa<UndefValue>(V) &&
+                                    Idx == UndefMaskElem) ||
+                                   (Idx != UndefMaskElem && V == Scalars[Idx]);
+                          });
       };
       if (!ReorderIndices.empty()) {
         // TODO: implement matching if the nodes are just reordered, still can
diff --git a/llvm/test/Transforms/SLPVectorizer/X86/crash_undef_mask_elem.ll b/llvm/test/Transforms/SLPVectorizer/X86/crash_undef_mask_elem.ll
new file mode 100644
index 0000000000000..0561aa8309fcf
--- /dev/null
+++ b/llvm/test/Transforms/SLPVectorizer/X86/crash_undef_mask_elem.ll
@@ -0,0 +1,47 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -slp-vectorizer -S -o - -mtriple=x86_64 < %s | FileCheck %s
+
+define void @main(i32 %x, i1 %b) {
+; CHECK-LABEL: @main(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[B:%.*]], label [[L3:%.*]], label [[L1_THREAD:%.*]]
+; CHECK:       L1:
+; CHECK-NEXT:    br label [[L3]]
+; CHECK:       L1.thread:
+; CHECK-NEXT:    br label [[L3]]
+; CHECK:       L3:
+; CHECK-NEXT:    [[DOTPR21:%.*]] = phi i32 [ [[X:%.*]], [[ENTRY:%.*]] ], [ [[DOTPR21]], [[L1:%.*]] ], [ [[X]], [[L1_THREAD]] ]
+; CHECK-NEXT:    [[DOTOLD_PR15:%.*]] = phi i32 [ [[X]], [[ENTRY]] ], [ 0, [[L1]] ], [ [[X]], [[L1_THREAD]] ]
+; CHECK-NEXT:    [[H_1:%.*]] = phi i32 [ undef, [[ENTRY]] ], [ undef, [[L1]] ], [ undef, [[L1_THREAD]] ]
+; CHECK-NEXT:    [[I_2:%.*]] = phi i32 [ 1, [[ENTRY]] ], [ [[I_2]], [[L1]] ], [ 1, [[L1_THREAD]] ]
+; CHECK-NEXT:    [[K_3:%.*]] = phi i32 [ 0, [[ENTRY]] ], [ [[I_2]], [[L1]] ], [ 1, [[L1_THREAD]] ]
+; CHECK-NEXT:    [[G_1:%.*]] = phi i32 [ undef, [[ENTRY]] ], [ [[O_1:%.*]], [[L1]] ], [ undef, [[L1_THREAD]] ]
+; CHECK-NEXT:    [[M_3:%.*]] = phi i32 [ undef, [[ENTRY]] ], [ [[I_2]], [[L1]] ], [ 1, [[L1_THREAD]] ]
+; CHECK-NEXT:    [[O_1]] = phi i32 [ undef, [[ENTRY]] ], [ [[O_1]], [[L1]] ], [ undef, [[L1_THREAD]] ]
+; CHECK-NEXT:    br label [[L4:%.*]]
+; CHECK:       L4:
+; CHECK-NEXT:    br label [[L1]]
+;
+entry:
+  br i1 %b, label %L3, label %L1.thread
+
+L1:
+  br label %L3
+
+L1.thread:
+  br label %L3
+
+L3:
+  %.pr21 = phi i32 [ %x, %entry ], [ %.pr21, %L1 ], [ %x, %L1.thread ]
+  %.old.pr15 = phi i32 [ %x, %entry ], [ 0, %L1 ], [ %x, %L1.thread ]
+  %h.1 = phi i32 [ undef, %entry ], [ undef, %L1 ], [ undef, %L1.thread ]
+  %i.2 = phi i32 [ 1, %entry ], [ %i.2, %L1 ], [ 1, %L1.thread ]
+  %k.3 = phi i32 [ 0, %entry ], [ %i.2, %L1 ], [ 1, %L1.thread ]
+  %g.1 = phi i32 [ undef, %entry ], [ %o.1, %L1 ], [ undef, %L1.thread ]
+  %m.3 = phi i32 [ undef, %entry ], [ %i.2, %L1 ], [ 1, %L1.thread ]
+  %o.1 = phi i32 [ undef, %entry ], [ %o.1, %L1 ], [ undef, %L1.thread ]
+  br label %L4
+
+L4:
+  br label %L1
+}