diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -765,9 +765,9 @@
              (treatment == UndefinedSymbolTreatment::warning ||
               treatment == UndefinedSymbolTreatment::suppress)) {
     if (treatment == UndefinedSymbolTreatment::warning)
-      error("'-undefined warning' only valid with '-flat_namespace'");
+      fatal("'-undefined warning' only valid with '-flat_namespace'");
     else
-      error("'-undefined suppress' only valid with '-flat_namespace'");
+      fatal("'-undefined suppress' only valid with '-flat_namespace'");
     treatment = UndefinedSymbolTreatment::error;
   }
   return treatment;
diff --git a/lld/test/MachO/treat-undef-sym.s b/lld/test/MachO/treat-undef-sym.s
--- a/lld/test/MachO/treat-undef-sym.s
+++ b/lld/test/MachO/treat-undef-sym.s
@@ -43,10 +43,10 @@
 # ERROR-NEXT: >>> referenced by
 
 # INVAL-WARNING: error: '-undefined warning' only valid with '-flat_namespace'
-# INVAL-WARNING-NEXT: error: undefined symbol: _bar
+# INVAL-WARNING-NOT: error: undefined symbol: _bar
 
 # INVAL-SUPPRESS: error: '-undefined suppress' only valid with '-flat_namespace'
-# INVAL-SUPPRESS-NEXT: error: undefined symbol: _bar
+# INVAL-SUPPRESS-NOT: error: undefined symbol: _bar
 
 # WARNING: warning: undefined symbol: _bar
 # WARNING-NEXT: >>> referenced by