diff --git a/include/swift/AST/DiagnosticsFrontend.def b/include/swift/AST/DiagnosticsFrontend.def
index b9b2625c79a4c..5b38bd1bb6561 100644
--- a/include/swift/AST/DiagnosticsFrontend.def
+++ b/include/swift/AST/DiagnosticsFrontend.def
@@ -156,6 +156,9 @@ ERROR(error_load_resolved_plugin,none,
 NOTE(note_valid_swift_versions, none,
      "valid arguments to '-swift-version' are %0", (StringRef))
 
+ERROR(error_module_interface_requires_language_mode,none,
+      "emitting module interface files requires '-language-mode'", ())
+
 ERROR(error_mode_cannot_emit_dependencies,none,
       "this mode does not support emitting dependency files", ())
 ERROR(error_mode_cannot_emit_reference_dependencies,none,
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 8b8f4b419cba3..ef80371c7b39b 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -1034,6 +1034,9 @@ 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);
+    HadError = true;
   }
 
   if (auto A = Args.getLastArg(OPT_package_description_version)) {
diff --git a/test/Driver/createCompilerInvocation.swift b/test/Driver/createCompilerInvocation.swift
index 1f9bec4fe9c3a..f3e046da10f31 100644
--- a/test/Driver/createCompilerInvocation.swift
+++ b/test/Driver/createCompilerInvocation.swift
@@ -9,12 +9,13 @@
 // CHECK-NOWARN-NOT: warning
 
 // RUN: %swift-ide-test_plain -test-createCompilerInvocation \
-// RUN:   -module-name foo -emit-module -emit-module-path %t/foo.swiftmodule -emit-objc-header -emit-objc-header-path %t/foo.h -enable-library-evolution -emit-module-interface -emit-module-interface-path %t/foo.swiftinterface -emit-library -emit-tbd -emit-tbd-path %t/foo.tbd -emit-dependencies -serialize-diagnostics %s \
+// RUN:   -swift-version 5 -module-name foo -emit-module -emit-module-path %t/foo.swiftmodule -emit-objc-header -emit-objc-header-path %t/foo.h -enable-library-evolution -emit-module-interface -emit-module-interface-path %t/foo.swiftinterface -emit-library -emit-tbd -emit-tbd-path %t/foo.tbd -emit-dependencies -serialize-diagnostics %s \
 // RUN:   2>&1 | %FileCheck %s --check-prefix=NORMAL_ARGS --implicit-check-not="error: "
 // NORMAL_ARGS: Frontend Arguments BEGIN
 // NORMAL_ARGS-DAG: -o{{$}}
 // NORMAL_ARGS-DAG: foo-{{[a-z0-9]+}}.o
 // NORMAL_ARGS-DAG: -c{{$}}
+// NORMAL_ARGS-DAG: -swift-version
 // NORMAL_ARGS-DAG: -module-name
 // NORMAL_ARGS-DAG: -emit-module-path
 // NORMAL_ARGS-DAG: -emit-module-doc-path
@@ -26,7 +27,7 @@
 // NORMAL_ARGS: Frontend Arguments END
 
 // RUN: %swift-ide-test_plain -test-createCompilerInvocation -force-no-outputs \
-// RUN:   -module-name foo -emit-module -emit-module-path %t/foo.swiftmodule -emit-objc-header -emit-objc-header-path %t/foo.h -enable-library-evolution -emit-module-interface -emit-module-interface-path %t/foo.swiftinterface -emit-library -emit-tbd -emit-tbd-path %t/foo.tbd -emit-dependencies -serialize-diagnostics %s \
+// RUN:   -swift-version 5 -module-name foo -emit-module -emit-module-path %t/foo.swiftmodule -emit-objc-header -emit-objc-header-path %t/foo.h -enable-library-evolution -emit-module-interface -emit-module-interface-path %t/foo.swiftinterface -emit-library -emit-tbd -emit-tbd-path %t/foo.tbd -emit-dependencies -serialize-diagnostics %s \
 // RUN:   2>&1 > %t.nooutput_args
 // RUN: %FileCheck %s --check-prefix=NOOUTPUT_ARGS --implicit-check-not="error: " < %t.nooutput_args
 // RUN: %FileCheck %s --check-prefix=NOOUTPUT_ARGS_NEG --implicit-check-not="error: " < %t.nooutput_args
diff --git a/test/ModuleInterface/language_mode.swift b/test/ModuleInterface/language_mode.swift
new file mode 100644
index 0000000000000..d46e32b1eeb89
--- /dev/null
+++ b/test/ModuleInterface/language_mode.swift
@@ -0,0 +1,27 @@
+// RUN: %empty-directory(%t)
+
+// 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: 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 \
+// RUN:   -emit-module-interface-path %t.swiftinterface \
+// RUN:   -emit-module -o /dev/null -language-mode 5
+
+// RUN: %FileCheck %s --check-prefix CHECK-SWIFTINTERFACE < %t.swiftinterface
+
+// RUN: %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 -swift-version 5
+
+// RUN: %FileCheck %s --check-prefix CHECK-SWIFTINTERFACE < %t.swiftinterface
+
+// CHECK-SWIFTINTERFACE: swift-module-flags:
+// CHECK-SWIFTINTERFACE-SAME: -enable-library-evolution
+// CHECK-SWIFTINTERFACE-SAME: -module-name Test
+// CHECK-SWIFTINTERFACE-SAME: {{-swift-version|-language-mode}} 5