Index: llvm/lib/MC/MCWin64EH.cpp
===================================================================
--- llvm/lib/MC/MCWin64EH.cpp
+++ llvm/lib/MC/MCWin64EH.cpp
@@ -576,7 +576,13 @@
   if (CodeWordsMod)
     CodeWords++;
   uint32_t EpilogCount = info->EpilogMap.size();
-  bool ExtensionWord = EpilogCount > 31 || TotalCodeBytes > 124;
+  // If we need to signal a larger EpilogCount or TotalCodeBytes, we need to
+  // use the extension word, and leave those fields as zero in row1.
+  // If both EpilogCount and CodeWords actually are zero though (e.g. due to
+  // missing/incomplete .seh directives in assembly), we still need to include
+  // the extension word, as that is how the reader will interpret it.
+  bool ExtensionWord = EpilogCount > 31 || TotalCodeBytes > 124 ||
+                       (EpilogCount == 0 && CodeWords == 0);
   if (!ExtensionWord) {
     row1 |= (EpilogCount & 0x1F) << 22;
     row1 |= (CodeWords & 0x1F) << 27;
Index: llvm/test/MC/AArch64/seh.s
===================================================================
--- llvm/test/MC/AArch64/seh.s
+++ llvm/test/MC/AArch64/seh.s
@@ -1,6 +1,8 @@
 // This test checks that the SEH directives don't cause the assembler to fail.
+// Checking that llvm-readobj doesn't bail out on the unwind data, but not
+// really checking the contents yet.
 
-// RUN: llvm-mc -triple aarch64-pc-win32 -filetype=obj %s | llvm-readobj -S -r - | FileCheck %s
+// RUN: llvm-mc -triple aarch64-pc-win32 -filetype=obj %s | llvm-readobj -S -r -u - | FileCheck %s
 
 // CHECK:      Sections [
 // CHECK:        Section {
@@ -15,7 +17,7 @@
 // CHECK-NEXT:   }
 // CHECK:        Section {
 // CHECK:          Name: .xdata
-// CHECK:          RawDataSize: 20
+// CHECK:          RawDataSize: 24
 // CHECK:          RelocationCount: 1
 // CHECK:          Characteristics [
 // CHECK-NEXT:       ALIGN_4BYTES