diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -462,7 +462,8 @@
 
   auto PrevCopyOperands = isCopyInstr(*PrevCopy, *TII, UseCopyInstr);
   // Check that the existing copy uses the correct sub registers.
-  if (PrevCopyOperands->Destination->isDead())
+  if (PrevCopyOperands->Destination->isDead() ||
+      PrevCopyOperands->Source->isUndef())
     return false;
   if (!isNopCopy(*PrevCopy, Src, Def, TRI, TII, UseCopyInstr))
     return false;
diff --git a/llvm/test/CodeGen/X86/machine-copy-prop.mir b/llvm/test/CodeGen/X86/machine-copy-prop.mir
--- a/llvm/test/CodeGen/X86/machine-copy-prop.mir
+++ b/llvm/test/CodeGen/X86/machine-copy-prop.mir
@@ -14,6 +14,7 @@
   define void @nocopyprop3() { ret void }
   define void @nocopyprop4() { ret void }
   define void @nocopyprop5() { ret void }
+  define void @nocopyprop6() { ret void }
 ...
 ---
 # The second copy is redundant and will be removed, check that we also remove
@@ -213,3 +214,24 @@
     $rip = COPY $rax
     $rip = COPY $rax
 ...
+---
+# The copies are obviously redundant, but the earlier COPY copies from "undef".
+# That copy may be lowered to a KILL instead and thus we cannot remove the
+# second COPY, otherwise we risk making $rax undef.
+# CHECK-LABEL: name: nocopyprop6
+# CHECK: bb.0:
+# CHECK:      $rax = COPY undef $rdi
+# CHECK-NEXT: NOOP implicit killed $rax
+# CHECK-NEXT: $rax = COPY $rdi
+# CHECK-NEXT: NOOP implicit $rax, implicit $rdi
+name: nocopyprop6
+tracksRegLiveness: true
+body: |
+  bb.0:
+    liveins: $rdi
+
+    $rax = COPY undef $rdi
+    NOOP implicit killed $rax
+    $rax = COPY $rdi
+    NOOP implicit $rax, implicit $rdi
+...