diff --git a/lib/CodeGen/RegisterCoalescer.cpp b/lib/CodeGen/RegisterCoalescer.cpp
index 0dc5ae188eb..535e5bd7628 100644
--- a/lib/CodeGen/RegisterCoalescer.cpp
+++ b/lib/CodeGen/RegisterCoalescer.cpp
@@ -935,7 +935,14 @@ RegisterCoalescer::removeCopyByCommutingDef(const CoalescerPair &CP,
     const SlotIndexes &Indexes = *LIS->getSlotIndexes();
     for (LiveInterval::SubRange &SA : IntA.subranges()) {
       VNInfo *ASubValNo = SA.getVNInfoAt(AIdx);
-      assert(ASubValNo != nullptr);
+      // Even if we are dealing with a full copy, some lanes can
+      // still be undefined.
+      // E.g.,
+      // undef A.subLow = ...
+      // B = COPY A <== A.subHigh is undefined here and does
+      //                not have a value number.
+      if (!ASubValNo)
+        continue;
       MaskA |= SA.LaneMask;
 
       IntB.refineSubRanges(
diff --git a/test/CodeGen/SystemZ/regcoal-undef-lane-4-rm-cp-commuting-def.mir b/test/CodeGen/SystemZ/regcoal-undef-lane-4-rm-cp-commuting-def.mir
new file mode 100644
index 00000000000..4245dafb00d
--- /dev/null
+++ b/test/CodeGen/SystemZ/regcoal-undef-lane-4-rm-cp-commuting-def.mir
@@ -0,0 +1,131 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -run-pass simple-register-coalescing %s -systemz-subreg-liveness -O3 -mtriple s390x-ibm-linux -mcpu z13 -o - | FileCheck %s
+#
+# When the coalescing tries to coalesce `%20 = COPY %18`,
+# %18 has been coalesced all the way up to %15 = ROSBG.
+# ROSBG has tied operands and can be commuted, thus it
+# maybe possible to swap the arguments of ROSBG to coalesce
+# this copy. That's what the coalescer attempts.
+#
+# This used to produce an assertion failure in the coalescer,
+# because this particular coalescing trick is used only on
+# full copy. Therefore we were asserting that all the subranges
+# at the point of the copy had a value number.
+# This is actually not true in that example, because %18 is
+# only partial defined before being fully copied.
+#
+# PR40215.
+---
+name:            main
+alignment:       4
+tracksRegLiveness: true
+machineFunctionInfo: {}
+body:             |
+  ; CHECK-LABEL: name: main
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.1(0x80000000)
+  ; CHECK:   [[LGHI:%[0-9]+]]:addr64bit = LGHI -30
+  ; CHECK:   [[LHIMux:%[0-9]+]]:grx32bit = LHIMux 1
+  ; CHECK:   [[LHIMux1:%[0-9]+]]:grx32bit = LHIMux 0
+  ; CHECK:   undef %20.subreg_l32:gr64bit = IMPLICIT_DEF
+  ; CHECK: bb.1:
+  ; CHECK:   successors: %bb.3(0x00000001), %bb.4(0x7fffffff)
+  ; CHECK:   CHIMux [[LHIMux]], 0, implicit-def $cc
+  ; CHECK:   BRC 14, 6, %bb.3, implicit killed $cc
+  ; CHECK:   J %bb.4
+  ; CHECK: bb.2:
+  ; CHECK:   successors:
+  ; CHECK:   STMux %20.subreg_l32, undef %8:addr64bit, 0, $noreg :: (store 4 into `i32* undef`)
+  ; CHECK: bb.3:
+  ; CHECK:   successors:
+  ; CHECK: bb.4:
+  ; CHECK:   successors: %bb.5(0x30000000), %bb.6(0x50000000)
+  ; CHECK:   [[LAY:%[0-9]+]]:gr64bit = LAY [[LGHI]], 19838, $noreg
+  ; CHECK:   [[LAY1:%[0-9]+]]:gr64bit = LAY [[LGHI]], 19840, $noreg
+  ; CHECK:   [[LAY2:%[0-9]+]]:gr64bit = LAY [[LGHI]], 19844, $noreg
+  ; CHECK:   [[OGR:%[0-9]+]]:gr64bit = OGR [[OGR]], [[LAY]], implicit-def dead $cc
+  ; CHECK:   undef %19.subreg_l32:gr64bit = AHIMuxK [[LGHI]].subreg_l32, 19843, implicit-def dead $cc
+  ; CHECK:   [[ROSBG:%[0-9]+]]:gr64bit = ROSBG [[ROSBG]], [[OGR]], 32, 63, 0, implicit-def dead $cc
+  ; The coalescer should have been able to swap the operands of
+  ; the OR, hence eliminating the copy of %20 and %18.
+  ; This is also visible here because the ROSBG operand was LHS
+  ; before the coalescer, now it is RHS.
+  ; CHECK:   %20.subreg_l32:gr64bit = OR %20.subreg_l32, [[ROSBG]].subreg_l32, implicit-def dead $cc
+  ; CHECK:   [[ROSBG1:%[0-9]+]]:gr64bit = ROSBG [[ROSBG1]], [[LAY1]], 32, 63, 0, implicit-def dead $cc
+  ; CHECK:   CHIMux [[LHIMux1]], 0, implicit-def $cc
+  ; CHECK:   BRC 14, 6, %bb.6, implicit killed $cc
+  ; CHECK:   J %bb.5
+  ; CHECK: bb.5:
+  ; CHECK:   successors: %bb.6(0x80000000)
+  ; CHECK: bb.6:
+  ; CHECK:   successors: %bb.2(0x00000001), %bb.7(0x7fffffff)
+  ; CHECK:   [[LGHI]]:addr64bit = nuw nsw LA [[LGHI]], 6, $noreg
+  ; CHECK:   CGHI [[LGHI]], 0, implicit-def $cc
+  ; CHECK:   BRC 14, 8, %bb.2, implicit killed $cc
+  ; CHECK: bb.7:
+  ; CHECK:   successors: %bb.1(0x80000000)
+  ; CHECK:   J %bb.1
+  bb.0:
+    %6:gr64bit = LGHI -30
+    %8:grx32bit = LHIMux 1
+    %21:grx32bit = LHIMux 0
+    %23:addr64bit = COPY killed %6
+    %24:gr32bit = IMPLICIT_DEF
+
+  bb.1:
+    successors: %bb.3(0x00000001), %bb.4(0x7fffffff)
+
+    %1:gr32bit = COPY killed %24
+    %0:addr64bit = COPY killed %23
+    CHIMux %8, 0, implicit-def $cc
+    BRC 14, 6, %bb.3, implicit killed $cc
+    J %bb.4
+
+  bb.2:
+    successors:
+
+    STMux killed %4, undef %22:addr64bit, 0, $noreg :: (store 4 into `i32* undef`)
+
+  bb.3:
+    successors:
+
+
+  bb.4:
+    successors: %bb.5(0x30000000), %bb.6(0x50000000)
+
+    %2:gr64bit = LAY %0, 19838, $noreg
+    %3:gr64bit = LAY %0, 19840, $noreg
+    %9:gr64bit = LAY %0, 19844, $noreg
+    %10:gr64bit = COPY killed %9
+    %10:gr64bit = OGR %10, killed %2, implicit-def dead $cc
+    %11:gr32bit = COPY %0.subreg_l32
+    %12:gr32bit = AHIMuxK killed %11, 19843, implicit-def dead $cc
+    undef %13.subreg_l32:gr64bit = COPY killed %12
+    %15:gr64bit = COPY killed %13
+    %15:gr64bit = ROSBG %15, killed %10, 32, 63, 0, implicit-def dead $cc
+    %16:gr32bit = COPY killed %15.subreg_l32
+    %17:gr32bit = COPY killed %16
+    %17:gr32bit = OR %17, killed %1, implicit-def dead $cc
+    undef %18.subreg_l32:gr64bit = COPY killed %17
+    %20:gr64bit = COPY killed %18
+    %20:gr64bit = ROSBG %20, killed %3, 32, 63, 0, implicit-def dead $cc
+    CHIMux %21, 0, implicit-def $cc
+    BRC 14, 6, %bb.6, implicit killed $cc
+    J %bb.5
+
+  bb.5:
+
+  bb.6:
+    successors: %bb.2(0x00000001), %bb.7(0x7fffffff)
+
+    %4:grx32bit = COPY killed %20.subreg_l32
+    %5:gr64bit = nuw nsw LA killed %0, 6, $noreg
+    CGHI %5, 0, implicit-def $cc
+    BRC 14, 8, %bb.2, implicit killed $cc
+
+  bb.7:
+    %23:addr64bit = COPY killed %5
+    %24:gr32bit = COPY killed %4
+    J %bb.1
+
+...