diff --git a/packages/dart_firebase_admin/lib/src/utils/utils.dart b/packages/dart_firebase_admin/lib/src/utils/utils.dart
index fa9444ea..9ecf51a6 100644
--- a/packages/dart_firebase_admin/lib/src/utils/utils.dart
+++ b/packages/dart_firebase_admin/lib/src/utils/utils.dart
@@ -20,35 +20,38 @@ String get dartVersion =>
 
 /// Generates the update mask for the provided object.
 /// Note this will ignore the last key with value `null`.
-List<String> generateUpdateMask(
-  Object? obj, {
-  List<String> terminalPaths = const [],
-  String root = '',
-}) {
+List<String> _generateUpdateMask(Object? obj, String root) {
   if (obj is! Map) return [];
 
   final updateMask = <String>[];
   for (final key in obj.keys) {
-    final nextPath = root.isEmpty ? '$root.$key' : '$key';
+    final nextPath = root.isEmpty ? key.toString() : '$root.$key';
     // We hit maximum path.
     // Consider switching to Set<string> if the list grows too large.
-    if (terminalPaths.contains(nextPath)) {
-      // Add key and stop traversing this branch.
-      updateMask.add('$key');
-    } else {
-      final maskList = generateUpdateMask(
-        obj[key],
-        terminalPaths: terminalPaths,
-        root: nextPath,
-      );
-      if (maskList.isNotEmpty) {
-        for (final mask in maskList) {
-          updateMask.add('$key.$mask');
-        }
-      } else {
-        updateMask.add('$key');
+    final maskList = _generateUpdateMask(obj[key], nextPath);
+    if (maskList.isNotEmpty) {
+      for (final mask in maskList) {
+        updateMask.add('$key.$mask');
       }
+    } else {
+      updateMask.add('$key');
     }
   }
   return updateMask;
 }
+
+/// Generates a list of field paths (update mask) for the provided object.
+///
+/// Returns an empty list if the [obj] is not a [Map].
+///
+/// All keys present in the map are included in the mask. If a key's value is
+/// another map, the paths are extended to the keys of that map. If a key's
+/// value is not a map (including primitive values like numbers, strings, and
+/// `null`), that key becomes a leaf in the path.
+///
+/// Example:
+/// ```dart
+/// generateUpdateMask({'a': 1, 'b': {'c': null, 'd': {}}})
+/// // Returns: ['a', 'b.c', 'b.d']
+/// ```
+List<String> generateUpdateMask(Object? obj) => _generateUpdateMask(obj, '');
diff --git a/packages/dart_firebase_admin/test/unit/utils_test.dart b/packages/dart_firebase_admin/test/unit/utils_test.dart
new file mode 100644
index 00000000..ba379dbb
--- /dev/null
+++ b/packages/dart_firebase_admin/test/unit/utils_test.dart
@@ -0,0 +1,62 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import 'package:dart_firebase_admin/src/utils/utils.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('generateUpdateMask', () {
+    test('non-map objects', () {
+      expect(generateUpdateMask(null), isEmpty);
+      expect(generateUpdateMask(1), isEmpty);
+      expect(generateUpdateMask('string'), isEmpty);
+      expect(generateUpdateMask([]), isEmpty);
+    });
+
+    test('empty map', () {
+      final obj = <String, dynamic>{};
+      expect(generateUpdateMask(obj), isEmpty);
+    });
+
+    test('flat map', () {
+      final obj = {'a': 1, 'b': 'string', 'c': null};
+      expect(generateUpdateMask(obj), containsAll(['a', 'b', 'c']));
+    });
+
+    test('nested maps', () {
+      final obj = {
+        'a': 1,
+        'b': {'c': 2, 'd': null},
+      };
+      expect(generateUpdateMask(obj), containsAll(['a', 'b.c', 'b.d']));
+    });
+
+    test('deeply nested maps', () {
+      final obj = {
+        'a': {
+          'b': {'c': null},
+        },
+      };
+      expect(generateUpdateMask(obj), containsAll(['a.b.c']));
+    });
+
+    test('empty maps as leaf nodes', () {
+      final obj = {
+        'a': <String, dynamic>{},
+        'b': {'c': <String, dynamic>{}},
+      };
+      expect(generateUpdateMask(obj), containsAll(['a', 'b.c']));
+    });
+  });
+}