diff --git a/lld/MachO/ICF.cpp b/lld/MachO/ICF.cpp
--- a/lld/MachO/ICF.cpp
+++ b/lld/MachO/ICF.cpp
@@ -114,7 +114,8 @@
       if (sa->kind() != sb->kind())
         return false;
       if (!isa<Defined>(sa)) {
-        assert(isa<DylibSymbol>(sa));
+        // ICF runs before Undefineds are reported.
+        assert(isa<DylibSymbol>(sa) || isa<Undefined>(sa));
         return sa == sb;
       }
       const auto *da = cast<Defined>(sa);
@@ -275,7 +276,7 @@
             } else {
               hash += defined->value;
             }
-          } else if (!isa<Undefined>(sym))
+          } else if (!isa<Undefined>(sym)) // ICF runs before Undefined diags.
             llvm_unreachable("foldIdenticalSections symbol kind");
         }
       }
diff --git a/lld/test/MachO/invalid/undefined-symbol.s b/lld/test/MachO/invalid/undefined-symbol.s
--- a/lld/test/MachO/invalid/undefined-symbol.s
+++ b/lld/test/MachO/invalid/undefined-symbol.s
@@ -20,9 +20,17 @@
   retq
 
 #--- main.s
-.globl _main
 .text
+
+_anotherref:
+  callq _foo
+  movq $0, %rax
+  retq
+
+.globl _main
 _main:
   callq _foo
   movq $0, %rax
   retq
+
+.subsections_via_symbols