diff --git a/lld/MachO/Writer.cpp b/lld/MachO/Writer.cpp
--- a/lld/MachO/Writer.cpp
+++ b/lld/MachO/Writer.cpp
@@ -1102,10 +1102,16 @@
   if (config->entry && !isa<Undefined>(config->entry))
     prepareBranchTarget(config->entry);
   scanRelocations();
+
+  // Do not proceed if there was an undefined symbol.
+  if (errorCount())
+    return;
+
   if (in.stubHelper->isNeeded())
     in.stubHelper->setup();
   scanSymbols();
   createOutputSections<LP>();
+
   // After this point, we create no new segments; HOWEVER, we might
   // yet create branch-range extension thunks for architectures whose
   // hardware call instructions have limited range, e.g., ARM(64).
diff --git a/lld/test/MachO/invalid/arm64-thunk-undefined.s b/lld/test/MachO/invalid/arm64-thunk-undefined.s
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/invalid/arm64-thunk-undefined.s
@@ -0,0 +1,24 @@
+# REQUIRES: aarch64
+
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t.o
+## This shouldn't assert.
+# RUN: not %lld -arch arm64 -lSystem -o %t/thunk %t.o 2>&1 | FileCheck %s
+
+# CHECK: error: undefined symbol: _g
+
+.subsections_via_symbols
+
+.p2align 2
+
+.globl _main, _g
+
+.globl _main
+_main:
+  bl _g
+  ret
+
+_filler1:
+.space 0x4000000
+
+_filler2:
+.space 0x4000000