diff --git a/include/clang/Basic/DiagnosticDriverKinds.td b/include/clang/Basic/DiagnosticDriverKinds.td
index b14a3b6..ed49006 100644
--- a/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/include/clang/Basic/DiagnosticDriverKinds.td
@@ -113,7 +113,6 @@
   "cannot recognize the type of the toolchain">;
 
 def warn_O4_is_O3 : Warning<"-O4 is equivalent to -O3">, InGroup<Deprecated>;
-def warn_drv_invalid_value : Warning<"optimization level '%0' is unsupported; using '%1%2' instead.">;
 def warn_c_kext : Warning<
   "ignoring -fapple-kext which is valid for C++ and Objective-C++ only">;
 def warn_drv_input_file_unused : Warning<
diff --git a/lib/Driver/Tools.cpp b/lib/Driver/Tools.cpp
index 87ec763..c69ad53 100644
--- a/lib/Driver/Tools.cpp
+++ b/lib/Driver/Tools.cpp
@@ -2727,7 +2727,7 @@
   // preprocessed inputs and configure concludes that -fPIC is not supported.
   Args.ClaimAllArgs(options::OPT_D);
 
-  // Manually translate -O4 to -O3; let clang fall back on -O3 for others
+  // Manually translate -O4 to -O3; let clang reject others.
   if (Arg *A = Args.getLastArg(options::OPT_O_Group)) {
     if (A->getOption().matches(options::OPT_O4)) {
       CmdArgs.push_back("-O3");
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index d02e4a3..98745ac 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -299,14 +299,14 @@
   using namespace options;
   bool Success = true;
 
-  Opts.OptimizationLevel = getOptimizationLevel(Args, IK, Diags);
-  unsigned MaxOptLevel = 3;
-  if (Opts.OptimizationLevel > MaxOptLevel) {
-    // If the optimization level is not supported, fall back on the default optimization
-    Diags.Report(diag::warn_drv_invalid_value)
-        << Args.getLastArg(OPT_O)->getAsString(Args) << "-O" << MaxOptLevel;
-    Opts.OptimizationLevel = MaxOptLevel;
+  unsigned OptLevel = getOptimizationLevel(Args, IK, Diags);
+  if (OptLevel > 3) {
+    Diags.Report(diag::err_drv_invalid_value)
+      << Args.getLastArg(OPT_O)->getAsString(Args) << OptLevel;
+    OptLevel = 3;
+    Success = false;
   }
+  Opts.OptimizationLevel = OptLevel;
 
   // We must always run at least the always inlining pass.
   Opts.setInlining(
diff --git a/test/Driver/clang_f_opts.c b/test/Driver/clang_f_opts.c
index 462b11a..e837583 100644
--- a/test/Driver/clang_f_opts.c
+++ b/test/Driver/clang_f_opts.c
@@ -100,9 +100,6 @@
 // CHECK-MAX-O: warning: -O4 is equivalent to -O3
 // CHECK-MAX-O: -O3
 
-// RUN: %clang -### -S -O20 %s 2>&1 | FileCheck -check-prefix=CHECK-INVALID-O %s
-// CHECK-INVALID-O: warning: optimization level '-O20' is unsupported; using '-O3' instead.
-
 // Test that we don't error on these.
 // RUN: %clang -### -S -Werror                                                \
 // RUN:     -falign-functions -falign-functions=2 -fno-align-functions        \