diff --git a/json_serializable/lib/src/field_helpers.dart b/json_serializable/lib/src/field_helpers.dart
index 4539c7e5..c2de4ca5 100644
--- a/json_serializable/lib/src/field_helpers.dart
+++ b/json_serializable/lib/src/field_helpers.dart
@@ -58,9 +58,9 @@ class _FieldSet implements Comparable<_FieldSet> {
     /// preference for the getter if it's defined.
     int offsetFor(FieldElement2 e) {
       if (e.isSynthetic) {
-        return (e.getter2 ?? e.setter2)!.firstFragment.nameOffset2!;
+        return (e.getter2 ?? e.setter2)!.firstFragment.nameOffset2 ?? 0;
       }
-      return e.firstFragment.nameOffset2!;
+      return e.firstFragment.nameOffset2 ?? 0;
     }
 
     return offsetFor(a).compareTo(offsetFor(b));
diff --git a/json_serializable/test/integration/json_test_example.dart b/json_serializable/test/integration/json_test_example.dart
index 963cdc27..5b6a5581 100644
--- a/json_serializable/test/integration/json_test_example.dart
+++ b/json_serializable/test/integration/json_test_example.dart
@@ -266,3 +266,34 @@ class RegressionTestIssue1210 with RegressionTestIssue1210Mixin {
 
   Map<String, dynamic> toJson() => _$RegressionTestIssue1210ToJson(this);
 }
+
+@JsonSerializable()
+class CustomList extends ListBase<String> {
+  // Regression test for issue:
+  // https://github.com/google/json_serializable.dart/issues/1512
+
+  final List<String> _innerList = [];
+
+  CustomList();
+
+  @override
+  int get length => _innerList.length;
+
+  @override
+  set length(int newLength) {
+    _innerList.length = newLength;
+  }
+
+  @override
+  String operator [](int index) => _innerList[index];
+
+  @override
+  void operator []=(int index, String value) {
+    _innerList[index] = value;
+  }
+
+  factory CustomList.fromJson(Map<String, dynamic> json) =>
+      _$CustomListFromJson(json);
+
+  Map<String, dynamic> toJson() => _$CustomListToJson(this);
+}
diff --git a/json_serializable/test/integration/json_test_example.g.dart b/json_serializable/test/integration/json_test_example.g.dart
index 1f1b456e..66b12fa7 100644
--- a/json_serializable/test/integration/json_test_example.g.dart
+++ b/json_serializable/test/integration/json_test_example.g.dart
@@ -229,3 +229,15 @@ RegressionTestIssue1210 _$RegressionTestIssue1210FromJson(
 Map<String, dynamic> _$RegressionTestIssue1210ToJson(
   RegressionTestIssue1210 instance,
 ) => <String, dynamic>{'field': instance.field};
+
+CustomList _$CustomListFromJson(Map<String, dynamic> json) => CustomList()
+  ..first = json['first'] as String
+  ..last = json['last'] as String
+  ..length = (json['length'] as num).toInt();
+
+Map<String, dynamic> _$CustomListToJson(CustomList instance) =>
+    <String, dynamic>{
+      'first': instance.first,
+      'last': instance.last,
+      'length': instance.length,
+    };
diff --git a/json_serializable/test/integration/json_test_example.g_any_map.dart b/json_serializable/test/integration/json_test_example.g_any_map.dart
index f499eb13..d4458416 100644
--- a/json_serializable/test/integration/json_test_example.g_any_map.dart
+++ b/json_serializable/test/integration/json_test_example.g_any_map.dart
@@ -266,3 +266,34 @@ class RegressionTestIssue1210 with RegressionTestIssue1210Mixin {
 
   Map<String, dynamic> toJson() => _$RegressionTestIssue1210ToJson(this);
 }
+
+@JsonSerializable(anyMap: true)
+class CustomList extends ListBase<String> {
+  // Regression test for issue:
+  // https://github.com/google/json_serializable.dart/issues/1512
+
+  final List<String> _innerList = [];
+
+  CustomList();
+
+  @override
+  int get length => _innerList.length;
+
+  @override
+  set length(int newLength) {
+    _innerList.length = newLength;
+  }
+
+  @override
+  String operator [](int index) => _innerList[index];
+
+  @override
+  void operator []=(int index, String value) {
+    _innerList[index] = value;
+  }
+
+  factory CustomList.fromJson(Map<String, dynamic> json) =>
+      _$CustomListFromJson(json);
+
+  Map<String, dynamic> toJson() => _$CustomListToJson(this);
+}
diff --git a/json_serializable/test/integration/json_test_example.g_any_map.g.dart b/json_serializable/test/integration/json_test_example.g_any_map.g.dart
index f25c7971..fdcd10b2 100644
--- a/json_serializable/test/integration/json_test_example.g_any_map.g.dart
+++ b/json_serializable/test/integration/json_test_example.g_any_map.g.dart
@@ -227,3 +227,15 @@ RegressionTestIssue1210 _$RegressionTestIssue1210FromJson(Map json) =>
 Map<String, dynamic> _$RegressionTestIssue1210ToJson(
   RegressionTestIssue1210 instance,
 ) => <String, dynamic>{'field': instance.field};
+
+CustomList _$CustomListFromJson(Map json) => CustomList()
+  ..first = json['first'] as String
+  ..last = json['last'] as String
+  ..length = (json['length'] as num).toInt();
+
+Map<String, dynamic> _$CustomListToJson(CustomList instance) =>
+    <String, dynamic>{
+      'first': instance.first,
+      'last': instance.last,
+      'length': instance.length,
+    };