diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1659,7 +1659,6 @@
 
 // Determine if this is a vector of ConstantFPs and if so, return the minimal
 // type we can safely truncate all elements to.
-// TODO: Make these support undef elements.
 static Type *shrinkFPConstantVector(Value *V) {
   auto *CV = dyn_cast<Constant>(V);
   auto *CVVTy = dyn_cast<FixedVectorType>(V->getType());
@@ -1673,6 +1672,9 @@
   // For fixed-width vectors we find the minimal type by looking
   // through the constant values of the vector.
   for (unsigned i = 0; i != NumElts; ++i) {
+    if (isa<UndefValue>(CV->getAggregateElement(i)))
+      continue;
+
     auto *CFP = dyn_cast_or_null<ConstantFP>(CV->getAggregateElement(i));
     if (!CFP)
       return nullptr;
@@ -1688,7 +1690,7 @@
   }
 
   // Make a vector type from the minimal type.
-  return FixedVectorType::get(MinType, NumElts);
+  return MinType ? FixedVectorType::get(MinType, NumElts) : nullptr;
 }
 
 /// Find the minimum FP type we can safely truncate to.
diff --git a/llvm/test/Transforms/InstCombine/fpextend.ll b/llvm/test/Transforms/InstCombine/fpextend.ll
--- a/llvm/test/Transforms/InstCombine/fpextend.ll
+++ b/llvm/test/Transforms/InstCombine/fpextend.ll
@@ -83,12 +83,9 @@
 }
 
 ; Test with an undef element
-; TODO: Support undef elements.
 define <2 x float> @test6_undef(<2 x float> %x) nounwind  {
 ; CHECK-LABEL: @test6_undef(
-; CHECK-NEXT:    [[T1:%.*]] = fpext <2 x float> [[X:%.*]] to <2 x double>
-; CHECK-NEXT:    [[T3:%.*]] = fadd <2 x double> [[T1]], <double 0.000000e+00, double undef>
-; CHECK-NEXT:    [[T34:%.*]] = fptrunc <2 x double> [[T3]] to <2 x float>
+; CHECK-NEXT:    [[T34:%.*]] = fadd <2 x float> [[X:%.*]], <float 0.000000e+00, float undef>
 ; CHECK-NEXT:    ret <2 x float> [[T34]]
 ;
   %t1 = fpext <2 x float> %x to <2 x double>