diff --git a/lld/MachO/Symbols.h b/lld/MachO/Symbols.h
--- a/lld/MachO/Symbols.h
+++ b/lld/MachO/Symbols.h
@@ -306,8 +306,10 @@
          "Not a Symbol");
 
   bool isUsedInRegularObj = s->isUsedInRegularObj;
+  bool used = s->used;
   T *sym = new (s) T(std::forward<ArgT>(arg)...);
   sym->isUsedInRegularObj |= isUsedInRegularObj;
+  sym->used |= used;
   return sym;
 }
 
diff --git a/lld/MachO/Writer.cpp b/lld/MachO/Writer.cpp
--- a/lld/MachO/Writer.cpp
+++ b/lld/MachO/Writer.cpp
@@ -611,6 +611,7 @@
 
 static void prepareSymbolRelocation(Symbol *sym, const InputSection *isec,
                                     const Reloc &r) {
+  assert(sym->isLive());
   const RelocAttrs &relocAttrs = target->getRelocAttrs(r.type);
 
   if (relocAttrs.hasAttr(RelocAttrBits::BRANCH)) {
diff --git a/lld/test/MachO/dead-strip.s b/lld/test/MachO/dead-strip.s
--- a/lld/test/MachO/dead-strip.s
+++ b/lld/test/MachO/dead-strip.s
@@ -173,6 +173,18 @@
 # RUN: %lld -lSystem -dead_strip %t/strip-dylib-ref.o %t/dylib.dylib \
 # RUN:     -o %t/strip-dylib-ref -U _ref_undef_fun
 
+## Check that referenced undefs are kept with -undefined dynamic_lookup.
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos \
+# RUN:     %t/ref-undef.s -o %t/ref-undef.o
+# RUN: %lld -lSystem -dead_strip %t/ref-undef.o \
+# RUN:     -o %t/ref-undef -undefined dynamic_lookup
+# RUN: llvm-objdump --syms --lazy-bind %t/ref-undef | \
+# RUN:     FileCheck --check-prefix=STRIPDYNLOOKUP %s
+# STRIPDYNLOOKUP: SYMBOL TABLE:
+# STRIPDYNLOOKUP:   *UND* _ref_undef_fun
+# STRIPDYNLOOKUP: Lazy bind table:
+# STRIPDYNLOOKUP:   __DATA   __la_symbol_ptr {{.*}} flat-namespace _ref_undef_fun
+
 ## S_ATTR_LIVE_SUPPORT tests.
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos \
 # RUN:     %t/live-support.s -o %t/live-support.o
@@ -812,3 +824,9 @@
   .quad L._bar4
 
 .subsections_via_symbols
+
+#--- ref-undef.s
+.globl _main
+_main:
+  callq _ref_undef_fun
+.subsections_via_symbols