Index: clang/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- clang/lib/Frontend/CompilerInvocation.cpp
+++ clang/lib/Frontend/CompilerInvocation.cpp
@@ -649,8 +649,14 @@
   Opts.NoUseJumpTables = Args.hasArg(OPT_fno_jump_tables);
 
   Opts.PrepareForLTO = Args.hasArg(OPT_flto, OPT_flto_EQ);
-  const Arg *A = Args.getLastArg(OPT_flto, OPT_flto_EQ);
-  Opts.EmitSummaryIndex = A && A->containsValue("thin");
+  Opts.EmitSummaryIndex = false;
+  if (Arg *A = Args.getLastArg(OPT_flto_EQ)) {
+    StringRef S = A->getValue();
+    if (S == "thin")
+      Opts.EmitSummaryIndex = true;
+    else if (S != "full")
+      Diags.Report(diag::err_drv_invalid_value) << A->getAsString(Args) << S;
+  }
   Opts.LTOUnit = Args.hasFlag(OPT_flto_unit, OPT_fno_lto_unit, false);
   if (Arg *A = Args.getLastArg(OPT_fthinlto_index_EQ)) {
     if (IK.getLanguage() != InputKind::LLVM_IR)
Index: clang/test/CodeGen/thinlto-backend-option.ll
===================================================================
--- clang/test/CodeGen/thinlto-backend-option.ll
+++ clang/test/CodeGen/thinlto-backend-option.ll
@@ -8,6 +8,8 @@
 
 ; RUN: %clang -flto=thin -c -o %t.o %s
 ; RUN: llvm-lto -thinlto -o %t %t.o
-; RUN: not %clang_cc1 -x ir %t.o -fthinlto-index=%t.thinlto.bc -backend-option -nonexistent -emit-obj -o /dev/null 2>&1 | FileCheck %s
+; RUN: not %clang_cc1 -x ir %t.o -fthinlto-index=%t.thinlto.bc -backend-option -nonexistent -emit-obj -o /dev/null 2>&1 | FileCheck %s -check-prefix=UNKNOWN
+; UNKNOWN: clang: Unknown command line argument '-nonexistent'
 
-; CHECK: clang: Unknown command line argument '-nonexistent'
+; RUN: not %clang_cc1 -flto=thinfoo 2>&1 | FileCheck %s -check-prefix=INVALID
+; INVALID: error: invalid value 'thinfoo' in '-flto=thinfoo'