diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 8be441fe28e0..c5bf3dcac45d 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -10001,6 +10001,8 @@ static bool isTargetShuffleEquivalent(ArrayRef<int> Mask,
   int Size = Mask.size();
   if (Size != (int)ExpectedMask.size())
     return false;
+  assert(isUndefOrZeroOrInRange(ExpectedMask, 0, 2 * Size) &&
+         "Illegal target shuffle mask");
 
   for (int i = 0; i < Size; ++i)
     if (Mask[i] == SM_SentinelUndef)