Index: llvm/trunk/lib/LibDriver/LibDriver.cpp
===================================================================
--- llvm/trunk/lib/LibDriver/LibDriver.cpp
+++ llvm/trunk/lib/LibDriver/LibDriver.cpp
@@ -143,6 +143,13 @@
       });
       return 1;
     }
+    sys::fs::file_magic Magic =
+        sys::fs::identify_magic(MOrErr->Buf->getBuffer());
+    if (Magic != sys::fs::file_magic::coff_object &&
+        Magic != sys::fs::file_magic::bitcode) {
+      llvm::errs() << Arg->getValue() << ": not a COFF object or bitcode file\n";
+      return 1;
+    }
     Members.emplace_back(std::move(*MOrErr));
   }
 
Index: llvm/trunk/test/LibDriver/invalid.test
===================================================================
--- llvm/trunk/test/LibDriver/invalid.test
+++ llvm/trunk/test/LibDriver/invalid.test
@@ -0,0 +1,2 @@
+RUN: not llvm-lib %S/Inputs/cl-gl.obj 2>&1 | FileCheck %s
+CHECK: not a COFF object or bitcode file
Index: llvm/trunk/test/LibDriver/thin.test
===================================================================
--- llvm/trunk/test/LibDriver/thin.test
+++ llvm/trunk/test/LibDriver/thin.test
@@ -1,4 +1,4 @@
-RUN: echo foo > %t
+RUN: llvm-mc -triple=x86_64-pc-windows-msvc -filetype=obj -o %t %S/Inputs/a.s
 
 RUN: llvm-lib  -out:%t.a %t
 RUN: FileCheck --check-prefix=FAT %s < %t.a