diff --git a/llvm/lib/DebugInfo/DWARF/DWARFUnitIndex.cpp b/llvm/lib/DebugInfo/DWARF/DWARFUnitIndex.cpp
--- a/llvm/lib/DebugInfo/DWARF/DWARFUnitIndex.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFUnitIndex.cpp
@@ -118,7 +118,7 @@
     CASE(MACINFO);
     CASE(MACRO);
   }
-  llvm_unreachable("unknown DWARFSectionKind");
+  return "<unknown>";
 }
 
 void DWARFUnitIndex::dump(raw_ostream &OS) const {
diff --git a/llvm/test/DebugInfo/X86/debug-cu-index-unknown-section.s b/llvm/test/DebugInfo/X86/debug-cu-index-unknown-section.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/DebugInfo/X86/debug-cu-index-unknown-section.s
@@ -0,0 +1,33 @@
+# RUN: llvm-mc -triple x86_64-unknown-linux %s -filetype=obj -o - | \
+# RUN:   llvm-dwarfdump -debug-cu-index - | \
+# RUN:   FileCheck %s
+
+# CHECK:      .debug_cu_index contents:
+# CHECK-NEXT: version = 2 slots = 2
+# CHECK-EMPTY:
+# CHECK-NEXT: Index Signature          INFO                     <unknown>
+# CHECK-NEXT: ----- ------------------ ------------------------ ------------------------
+# CHECK-NEXT:     1 0x1100001122222222 [0x00001000, 0x00001010) [0x00002000, 0x00002020)
+
+    .section .debug_cu_index, "", @progbits
+## Header:
+    .long 2             # Version
+    .long 2             # Section count
+    .long 1             # Unit count
+    .long 2             # Slot count
+## Hash Table of Signatures:
+    .quad 0x1100001122222222
+    .quad 0
+## Parallel Table of Indexes:
+    .long 1
+    .long 0
+## Table of Section Offsets:
+## Row 0:
+    .long 1             # DW_SECT_INFO
+    .long 9             # Invalid section identifier
+# Row 1:
+    .long 0x1000        # Offset in .debug_info.dwo
+    .long 0x2000
+## Table of Section Sizes:
+    .long 0x10          # Size in .debug_info.dwo
+    .long 0x20