diff --git a/pkg/dev_compiler/tool/ddb b/pkg/dev_compiler/tool/ddb
index 4c95bf42acf..f52cc0368d5 100755
--- a/pkg/dev_compiler/tool/ddb
+++ b/pkg/dev_compiler/tool/ddb
@@ -14,8 +14,7 @@
 import 'dart:io';
 
 import 'package:args/args.dart' show ArgParser;
-import 'package:dev_compiler/src/compiler/js_names.dart'
-    as js_names;
+import 'package:dev_compiler/src/compiler/js_names.dart' as js_names;
 import 'package:path/path.dart' as p;
 
 enum NullSafety { strict, weak, disabled }
@@ -50,7 +49,8 @@ void main(List<String> args) async {
     ..addFlag('sound-null-safety',
         help: 'Compile for sound null safety at runtime. Passed through to the '
             'DDC binary. Defaults to false.',
-        negatable: true, defaultsTo: true)
+        negatable: true,
+        defaultsTo: true)
     ..addFlag('emit-debug-symbols',
         help: 'Pass through flag for DDC, emits debug symbols file along with '
             'the compiled module.',
@@ -62,7 +62,6 @@ void main(List<String> args) async {
     ..addFlag('native-null-assertions',
         help: 'Run with assertions on non-nullable values returned from native '
             'APIs.',
-        defaultsTo: true,
         negatable: true)
     ..addFlag('weak-null-safety-errors',
         help: 'Treat weak null safety warnings as errors.', defaultsTo: false)
@@ -126,7 +125,9 @@ void main(List<String> args) async {
   var soundNullSafety = options['sound-null-safety'] as bool;
   var emitDebugSymbols = options['emit-debug-symbols'] as bool;
   var nonNullAsserts = options['null-assertions'] as bool;
-  var nativeNonNullAsserts = options['native-null-assertions'] as bool;
+  var nativeNonNullAsserts = options.wasParsed('native-null-assertions')
+      ? options['native-null-assertions'] as bool
+      : soundNullSafety;
   var weakNullSafetyErrors = options['weak-null-safety-errors'] as bool;
   var canaryFeatures = options['canary'] as bool;
   var entry = p.canonicalize(options.rest.first);
diff --git a/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart b/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart
index bf903553ef9..d01fe03a8ed 100644
--- a/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart
+++ b/sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/types.dart
@@ -78,14 +78,20 @@ void nonNullAsserts(bool enable) {
 }
 
 @notNull
-bool _nativeNonNullAsserts = false;
+bool _nativeNonNullAsserts = compileTimeFlag('soundNullSafety');
 
-/// Enables null assertions on native APIs to make sure value returned from the
-/// browser is sound.
+/// Enables null assertions on native APIs to make sure values returned from the
+/// browser are sound.
 ///
 /// These apply to dart:html and similar web libraries. Note that these only are
 /// added in sound null-safety only.
 void nativeNonNullAsserts(bool enable) {
+  if (enable && !compileTimeFlag('soundNullSafety')) {
+    _warn('Enabling `native-null-assertions` is only supported when sound null '
+        'safety is enabled.');
+  }
+  // This value is only read from `checkNativeNonNull` and calls to that method
+  // are only generated in sound null safe code.
   _nativeNonNullAsserts = enable;
 }