diff --git a/llvm/include/llvm/IR/Instructions.h b/llvm/include/llvm/IR/Instructions.h
--- a/llvm/include/llvm/IR/Instructions.h
+++ b/llvm/include/llvm/IR/Instructions.h
@@ -2081,8 +2081,9 @@
   /// elements than its source vectors.
   /// Example: shufflevector <2 x n> A, <2 x n> B, <1,2,3>
   bool increasesLength() const {
-    unsigned NumSourceElts =
-        cast<FixedVectorType>(Op<0>()->getType())->getNumElements();
+    unsigned NumSourceElts = cast<VectorType>(Op<0>()->getType())
+                                 ->getElementCount()
+                                 .getKnownMinValue();
     unsigned NumMaskElts = ShuffleMask.size();
     return NumSourceElts < NumMaskElts;
   }
diff --git a/llvm/unittests/IR/InstructionsTest.cpp b/llvm/unittests/IR/InstructionsTest.cpp
--- a/llvm/unittests/IR/InstructionsTest.cpp
+++ b/llvm/unittests/IR/InstructionsTest.cpp
@@ -1083,7 +1083,19 @@
                             Constant::getNullValue(VScaleV4Int32Ty));
   int Index = 0;
   EXPECT_FALSE(Id13->isExtractSubvectorMask(Index));
+  EXPECT_FALSE(Id13->changesLength());
+  EXPECT_FALSE(Id13->increasesLength());
   delete Id13;
+
+  // Result has twice as many operands.
+  Type *VScaleV2Int32Ty = ScalableVectorType::get(Int32Ty, 2);
+  ShuffleVectorInst *Id14 =
+      new ShuffleVectorInst(Constant::getAllOnesValue(VScaleV2Int32Ty),
+                            UndefValue::get(VScaleV2Int32Ty),
+                            Constant::getNullValue(VScaleV4Int32Ty));
+  EXPECT_TRUE(Id14->changesLength());
+  EXPECT_TRUE(Id14->increasesLength());
+  delete Id14;
 }
 
 TEST(InstructionsTest, GetSplat) {