diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index bcbccf167c..0e5e93e181 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -12,8 +12,13 @@ void mergeAttributes(Map<String, dynamic> attributes,
     {@required Map<String, dynamic> into}) {
   assert(attributes != null && into != null);
   attributes.forEach((String name, dynamic value) {
-    final dynamic targetValue = into[name];
-    if (value is Map && targetValue is Map) {
+    dynamic targetValue = into[name];
+    if (value is Map) {
+      if (targetValue is! Map) {
+        // Let mergeAttributes make a deep copy, because assigning a reference
+        // of 'value' will expose 'value' to be mutated by further merges.
+        into[name] = targetValue = <String, dynamic>{};
+      }
       mergeAttributes(value, into: targetValue);
     } else {
       into[name] = value;
diff --git a/test/utils_test.dart b/test/utils_test.dart
index 3ee15130dc..e86a57a2aa 100644
--- a/test/utils_test.dart
+++ b/test/utils_test.dart
@@ -35,6 +35,25 @@ void main() {
         },
       });
     });
+
+    test('does not allow overriding original maps', () {
+      final environment = <String, dynamic>{
+        'extra': {
+          'device': 'Pixel 2',
+        },
+      };
+
+      final event = <String, dynamic>{
+        'extra': {
+          'widget': 'Scaffold',
+        },
+      };
+
+      final target = <String, dynamic>{};
+      mergeAttributes(environment, into: target);
+      mergeAttributes(event, into: target);
+      expect(environment['extra'], {'device': 'Pixel 2'});
+    });
   });
 
   group('formatDateAsIso8601WithSecondPrecision', () {