Index: ELF/Driver.cpp
===================================================================
--- ELF/Driver.cpp
+++ ELF/Driver.cpp
@@ -316,7 +316,7 @@
     link<ELF64BE>(Args);
     return;
   default:
-    error("-m or at least a .o file required");
+    error("output format unknown: -m or at least one .o file required");
   }
 }
 
Index: test/ELF/no-obj.s
===================================================================
--- test/ELF/no-obj.s
+++ test/ELF/no-obj.s
@@ -2,7 +2,7 @@
 // RUN: llvm-ar rcs %t.a %t.o
 // RUN: not ld.lld -o %t2 -u _start %t.a 2>&1 | FileCheck %s
 
-// CHECK: -m or at least a .o file required
+// CHECK: output format unknown: -m or at least one .o file required
 
 .global _start
 _start: