diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4136,7 +4136,8 @@
 
   // Deal with partial undef vector constants: select ?, VecC, VecC' --> VecC''
   Constant *TrueC, *FalseC;
-  if (TrueVal->getType()->isVectorTy() && match(TrueVal, m_Constant(TrueC)) &&
+  if (isa<FixedVectorType>(TrueVal->getType()) &&
+      match(TrueVal, m_Constant(TrueC)) &&
       match(FalseVal, m_Constant(FalseC))) {
     unsigned NumElts =
         cast<FixedVectorType>(TrueC->getType())->getNumElements();
diff --git a/llvm/test/Transforms/InstSimplify/select.ll b/llvm/test/Transforms/InstSimplify/select.ll
--- a/llvm/test/Transforms/InstSimplify/select.ll
+++ b/llvm/test/Transforms/InstSimplify/select.ll
@@ -957,3 +957,14 @@
   ret i32 %r1.sroa.0.1
 }
 declare i32 @llvm.cttz.i32(i32, i1 immarg)
+
+; Partial undef scalable vectors should be ignored.
+define <vscale x 2 x i1> @ignore_scalable_undef(<vscale x 2 x i1> %cond) {
+; CHECK-LABEL: @ignore_scalable_undef(
+; CHECK-NEXT:    [[S:%.*]] = select <vscale x 2 x i1> [[COND:%.*]], <vscale x 2 x i1> undef, <vscale x 2 x i1> insertelement (<vscale x 2 x i1> undef, i1 true, i32 0)
+; CHECK-NEXT:    ret <vscale x 2 x i1> [[S]]
+;
+  %vec = insertelement <vscale x 2 x i1> undef, i1 true, i32 0
+  %s = select <vscale x 2 x i1> %cond, <vscale x 2 x i1> undef, <vscale x 2 x i1> %vec
+  ret <vscale x 2 x i1> %s
+}