diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 2f8b9df386dcc..79534a755b6ab 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -1035,10 +1035,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     if (!isValid)
       diagnoseSwiftVersion(vers, A, Args, Diags);
   } else if (FrontendOpts.InputsAndOutputs.hasModuleInterfaceOutputPath()) {
-    Diags.diagnose({}, diag::error_module_interface_requires_language_mode)
-        .limitBehavior(DiagnosticBehavior::Warning);
-    // FIXME: Make this an error again (rdar://145168219)
-    // HadError = true;
+    Diags.diagnose({}, diag::error_module_interface_requires_language_mode);
+    HadError = true;
   }
 
   if (auto A = Args.getLastArg(OPT_package_description_version)) {
diff --git a/test/ModuleInterface/language_mode.swift b/test/ModuleInterface/language_mode.swift
index 4522c5fdd38ed..d46e32b1eeb89 100644
--- a/test/ModuleInterface/language_mode.swift
+++ b/test/ModuleInterface/language_mode.swift
@@ -1,11 +1,11 @@
 // RUN: %empty-directory(%t)
 
-// RUN: %swift_frontend_plain -target %target-swift-5.1-abi-triple %s \
+// RUN: not %swift_frontend_plain -target %target-swift-5.1-abi-triple %s \
 // RUN:   -enable-library-evolution -module-name Test \
 // RUN:   -emit-module-interface-path %t.swiftinterface \
 // RUN:   -emit-module -o /dev/null 2>&1 | %FileCheck %s
 
-// CHECK: <unknown>:0: warning: emitting module interface files requires '-language-mode'
+// CHECK: <unknown>:0: error: emitting module interface files requires '-language-mode'
 
 // RUN: %swift_frontend_plain -target %target-swift-5.1-abi-triple %s \
 // RUN:   -enable-library-evolution -module-name Test \