Index: lib/CodeGen/TargetLoweringObjectFileImpl.cpp
===================================================================
--- lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -232,7 +232,11 @@
   if (!MD)
     return nullptr;
 
-  auto *VM = dyn_cast<ValueAsMetadata>(MD->getOperand(0));
+  const MDOperand &Op = MD->getOperand(0);
+  if (!Op.get())
+    return nullptr;
+
+  auto *VM = dyn_cast<ValueAsMetadata>(Op);
   if (!VM)
     report_fatal_error("MD_associated operand is not ValueAsMetadata");
 
Index: test/CodeGen/X86/elf-associated.ll
===================================================================
--- test/CodeGen/X86/elf-associated.ll
+++ test/CodeGen/X86/elf-associated.ll
@@ -37,3 +37,8 @@
 @l = global i32 1, section "ccc", !associated !5
 !5 = !{i32* null}
 ; CHECK-DAG: .section	ccc,"aw",@progbits
+
+; Null metadata.
+@m = global i32 1, section "ddd", !associated !6
+!6 = distinct !{null}
+; CHECK-DAG: .section	ddd,"aw",@progbits