diff --git a/llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp b/llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp
--- a/llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp
+++ b/llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp
@@ -338,6 +338,9 @@
     Value *extractVector(unsigned I, unsigned J, unsigned NumElts,
                          IRBuilder<> &Builder) const {
       Value *Vec = isColumnMajor() ? getColumn(J) : getRow(I);
+      assert(cast<FixedVectorType>(Vec->getType())->getNumElements() >=
+                 NumElts &&
+             "Extracted vector will contain poison values");
       return Builder.CreateShuffleVector(
           Vec, createSequentialMask(isColumnMajor() ? I : J, NumElts, 0),
           "block");