diff --git a/.github/workflows/dart.yml b/.github/workflows/dart.yml
index 8b975e0..65727ed 100644
--- a/.github/workflows/dart.yml
+++ b/.github/workflows/dart.yml
@@ -12,15 +12,15 @@ jobs:
     runs-on: ubuntu-latest
 
     container:
-      image:  google/dart:latest
+      image:  google/dart:beta
 
     steps:
       - uses: actions/checkout@v2
       - name: Install dependencies
-        run: pub get
+        run: dart pub get
       - name: Format
         run: dartfmt --dry-run --set-exit-if-changed lib test example
       - name: Analyzer
-        run: dartanalyzer --fatal-infos --fatal-warnings lib test example
+        run: dart analyze --fatal-infos --fatal-warnings
       - name: Tests
-        run: pub run test_coverage --no-badge --print-test-output --min-coverage 100
+        run: dart run test_coverage --no-badge --print-test-output --min-coverage 100
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3bc1a9f..80c2221 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,7 @@
 ## [Unreleased]
+### Changed
+- Prepared for null safety
+
 ## [0.2.0] - 2020-09-07
 ### Added
 - Ability to create arrays and set adjacent indices
@@ -83,4 +86,4 @@ Previously, no modification would be made and no errors/exceptions thrown.
 [0.0.0+dev.4]: https://github.com/f3ath/jessie/compare/0.0.0+dev.3...0.0.0+dev.4
 [0.0.0+dev.3]: https://github.com/f3ath/jessie/compare/0.0.0+dev.2...0.0.0+dev.3
 [0.0.0+dev.2]: https://github.com/f3ath/jessie/compare/0.0.0+dev.1...0.0.0+dev.2
-[0.0.0+dev.1]: https://github.com/f3ath/jessie/compare/0.0.0+dev.0...0.0.0+dev.1
+[0.0.0+dev.1]: https://github.com/f3ath/jessie/compare/0.0.0+dev.0...0.0.0+dev.1
\ No newline at end of file
diff --git a/analysis_options.yaml b/analysis_options.yaml
index 7783dc8..d9406f3 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -2,4 +2,4 @@ include: package:pedantic/analysis_options.yaml
 linter:
   rules:
     - sort_constructors_first
-    - sort_unnamed_constructors_first
\ No newline at end of file
+    - sort_unnamed_constructors_first
diff --git a/lib/src/ast/tokenize.dart b/lib/src/ast/tokenize.dart
index d819535..25e10f2 100644
--- a/lib/src/ast/tokenize.dart
+++ b/lib/src/ast/tokenize.dart
@@ -1,5 +1,5 @@
 Iterable<String> tokenize(String expr) =>
-    _tokens.allMatches(expr).map((match) => match.group(0));
+    _tokens.allMatches(expr).map((match) => match.group(0).toString());
 
 final _tokens = RegExp([
   r'\$', // root
diff --git a/lib/src/json_path.dart b/lib/src/json_path.dart
index 03bbbd6..9175d81 100644
--- a/lib/src/json_path.dart
+++ b/lib/src/json_path.dart
@@ -15,11 +15,12 @@ class JsonPath {
   /// in the [expression].
   ///
   /// Throws [FormatException] if the [expression] can not be parsed.
-  factory JsonPath(String expression, {Map<String, Predicate> filter}) {
+  factory JsonPath(String expression,
+      {Map<String, Predicate> filter = const {}}) {
     if (expression.isEmpty) throw FormatException('Empty expression');
     ParsingState state = Ready(RootSelector());
     AST(tokenize(expression)).nodes.forEach((node) {
-      state = state.process(node, filter ?? {});
+      state = state.process(node, filter);
     });
     return JsonPath._(state.selector);
   }
@@ -33,7 +34,7 @@ class JsonPath {
       _selector.read([JsonPathMatch(json, '')]);
 
   /// Returns a copy of [json] with all matching values replaced with [value].
-  dynamic set(dynamic json, dynamic value) => _selector.set(json, (_) => value);
+  dynamic set(json, value) => _selector.set(json, (_) => value);
 
   @override
   String toString() => _selector.expression();
diff --git a/lib/src/parsing_state.dart b/lib/src/parsing_state.dart
index a37a2ab..af794bf 100644
--- a/lib/src/parsing_state.dart
+++ b/lib/src/parsing_state.dart
@@ -20,7 +20,7 @@ abstract class ParsingState {
 
 /// Ready to process the next node
 class Ready implements ParsingState {
-  Ready(this.selector);
+  const Ready(this.selector);
 
   @override
   final Selector selector;
@@ -63,10 +63,11 @@ class Ready implements ParsingState {
 
   Filter _filter(List<Node> nodes, Map<String, Predicate> filters) {
     final name = nodes[1].value;
-    if (!filters.containsKey(name)) {
+    final filter = filters[name];
+    if (filter == null) {
       throw FormatException('Filter not found: "${name}"');
     }
-    return Filter(name, filters[name]);
+    return Filter(name, filter);
   }
 
   bool _isFilter(List<Node> nodes) => nodes.first.value == '?';
@@ -83,9 +84,9 @@ class Ready implements ParsingState {
   }
 
   Slice _slice(List<Node> nodes) {
-    int first;
-    int last;
-    int step;
+    int? first;
+    int? last;
+    int? step;
     var colons = 0;
     nodes.forEach((node) {
       if (node.value == ':') {
@@ -107,7 +108,7 @@ class Ready implements ParsingState {
 }
 
 class AwaitingField implements ParsingState {
-  AwaitingField(this.selector);
+  const AwaitingField(this.selector);
 
   @override
   final Selector selector;
diff --git a/lib/src/selector/quote.dart b/lib/src/quote.dart
similarity index 100%
rename from lib/src/selector/quote.dart
rename to lib/src/quote.dart
diff --git a/lib/src/selector/filter.dart b/lib/src/selector/filter.dart
index 238371a..b6671b6 100644
--- a/lib/src/selector/filter.dart
+++ b/lib/src/selector/filter.dart
@@ -19,6 +19,6 @@ class Filter with SelectorMixin implements Selector {
   String expression() => '[?$name]';
 
   @override
-  dynamic set(dynamic json, Replacement replacement) =>
+  dynamic set(json, Replacement replacement) =>
       isApplicable(json) ? replacement(json) : json;
 }
diff --git a/lib/src/selector/joint.dart b/lib/src/selector/joint.dart
index 9102b63..427c400 100644
--- a/lib/src/selector/joint.dart
+++ b/lib/src/selector/joint.dart
@@ -34,6 +34,6 @@ class Joint with SelectorMixin implements Selector {
       right.expression();
 
   @override
-  dynamic set(dynamic json, Replacement replacement) =>
+  dynamic set(json, Replacement replacement) =>
       left.set(json, (_) => right.set(_, replacement));
 }
diff --git a/lib/src/selector/list_union.dart b/lib/src/selector/list_union.dart
index 7ca6c0a..af3664c 100644
--- a/lib/src/selector/list_union.dart
+++ b/lib/src/selector/list_union.dart
@@ -9,14 +9,14 @@ class ListUnion with SelectorMixin implements Selector {
 
   @override
   Iterable<JsonPathMatch> read(Iterable<JsonPathMatch> matches) => matches
-      .map((r) => (r.value is List) ? _map(r.value, r.path) : [])
+      .map((r) => (r.value is List) ? _map(r.value, r.path) : <JsonPathMatch>[])
       .expand((_) => _);
 
   @override
   String expression() => '[${_indices.join(',')}]';
 
   @override
-  dynamic set(dynamic json, Replacement replacement) {
+  dynamic set(json, Replacement replacement) {
     json ??= [];
     if (json is List) {
       json = [...json];
diff --git a/lib/src/selector/list_wildcard.dart b/lib/src/selector/list_wildcard.dart
index 4d72282..9abc2c4 100644
--- a/lib/src/selector/list_wildcard.dart
+++ b/lib/src/selector/list_wildcard.dart
@@ -13,10 +13,10 @@ class ListWildcard with SelectorMixin implements Selector {
   String expression() => '[*]';
 
   @override
-  dynamic set(dynamic json, Replacement replacement) =>
+  dynamic set(json, Replacement replacement) =>
       (json is List) ? json.map(replacement).toList() : json;
 
-  Iterable<JsonPathMatch> _wrap(List val, String path) sync* {
+  static Iterable<JsonPathMatch> _wrap(List val, String path) sync* {
     for (var i = 0; i < val.length; i++) {
       yield JsonPathMatch(val[i], path + '[$i]');
     }
diff --git a/lib/src/selector/object_union.dart b/lib/src/selector/object_union.dart
index 774f20f..32f91a1 100644
--- a/lib/src/selector/object_union.dart
+++ b/lib/src/selector/object_union.dart
@@ -1,5 +1,5 @@
 import 'package:json_path/src/json_path_match.dart';
-import 'package:json_path/src/selector/quote.dart';
+import 'package:json_path/src/quote.dart';
 import 'package:json_path/src/selector/selector.dart';
 import 'package:json_path/src/selector/selector_mixin.dart';
 
@@ -18,7 +18,7 @@ class ObjectUnion with SelectorMixin implements Selector {
   String expression() => '[${_keys.map((k) => Quote(k)).join(',')}]';
 
   @override
-  dynamic set(dynamic json, Replacement replacement) {
+  dynamic set(json, Replacement replacement) {
     if (json == null) return _patch(<String, dynamic>{}, replacement);
     if (json is Map) return {...json, ..._patch(json, replacement)};
     return json;
diff --git a/lib/src/selector/object_wildcard.dart b/lib/src/selector/object_wildcard.dart
index f665cee..08eb1a8 100644
--- a/lib/src/selector/object_wildcard.dart
+++ b/lib/src/selector/object_wildcard.dart
@@ -1,5 +1,5 @@
 import 'package:json_path/src/json_path_match.dart';
-import 'package:json_path/src/selector/quote.dart';
+import 'package:json_path/src/quote.dart';
 import 'package:json_path/src/selector/selector.dart';
 import 'package:json_path/src/selector/selector_mixin.dart';
 
@@ -24,7 +24,7 @@ class ObjectWildcard with SelectorMixin implements Selector {
       .map((e) => JsonPathMatch(e.value, path + '[${e.key}]'));
 
   @override
-  dynamic set(dynamic json, Replacement replacement) => (json is Map)
+  dynamic set(json, Replacement replacement) => (json is Map)
       ? json.map((key, value) => MapEntry(key, replacement(value)))
       : json;
 }
diff --git a/lib/src/selector/recursive.dart b/lib/src/selector/recursive.dart
index 029a9e2..d221576 100644
--- a/lib/src/selector/recursive.dart
+++ b/lib/src/selector/recursive.dart
@@ -1,5 +1,5 @@
 import 'package:json_path/src/json_path_match.dart';
-import 'package:json_path/src/selector/quote.dart';
+import 'package:json_path/src/quote.dart';
 import 'package:json_path/src/selector/selector.dart';
 import 'package:json_path/src/selector/selector_mixin.dart';
 
@@ -12,7 +12,7 @@ class Recursive with SelectorMixin implements Selector {
   String expression() => '..';
 
   @override
-  dynamic set(dynamic json, Replacement replacement) {
+  dynamic set(json, Replacement replacement) {
     if (json is Map) {
       return _replaceInMap(json, replacement);
     }
diff --git a/lib/src/selector/root.dart b/lib/src/selector/root.dart
index 0e9f32a..683db03 100644
--- a/lib/src/selector/root.dart
+++ b/lib/src/selector/root.dart
@@ -13,5 +13,5 @@ class RootSelector with SelectorMixin implements Selector {
   String expression() => r'$';
 
   @override
-  dynamic set(dynamic json, Replacement replacement) => replacement(json);
+  dynamic set(json, Replacement replacement) => replacement(json);
 }
diff --git a/lib/src/selector/selector.dart b/lib/src/selector/selector.dart
index 30fa731..ec74a15 100644
--- a/lib/src/selector/selector.dart
+++ b/lib/src/selector/selector.dart
@@ -12,7 +12,7 @@ abstract class Selector {
   Selector then(Selector other);
 
   /// Returns a copy of [json] with all selected values modified using [replacement] function.
-  dynamic set(dynamic json, Replacement replacement);
+  dynamic set(json, Replacement replacement);
 }
 
-typedef Replacement = dynamic Function(dynamic value);
+typedef Replacement<V, T> = V Function(T value);
diff --git a/lib/src/selector/slice.dart b/lib/src/selector/slice.dart
index 984da9a..ed47c4f 100644
--- a/lib/src/selector/slice.dart
+++ b/lib/src/selector/slice.dart
@@ -5,7 +5,7 @@ import 'package:json_path/src/selector/selector.dart';
 import 'package:json_path/src/selector/selector_mixin.dart';
 
 class Slice with SelectorMixin implements Selector {
-  Slice({int first, this.last, int step})
+  Slice({int? first, this.last, int? step})
       : first = first ?? 0,
         step = step ?? 1;
 
@@ -18,7 +18,7 @@ class Slice with SelectorMixin implements Selector {
 
   final int first;
 
-  final int last;
+  final int? last;
 
   final int step;
 
@@ -33,7 +33,7 @@ class Slice with SelectorMixin implements Selector {
       '[${first == 0 ? '' : first}:${last ?? ''}${step != 1 ? ':$step' : ''}]';
 
   @override
-  dynamic set(dynamic json, Replacement replacement) {
+  dynamic set(json, Replacement replacement) {
     if (json is List) {
       final indices = _indices(json);
       if (indices.isNotEmpty) {
@@ -57,7 +57,7 @@ class Slice with SelectorMixin implements Selector {
 
   int _actualLast(int len) {
     if (last == null) return len;
-    if (last < 0) return min(len, len + last);
-    return min(len, last);
+    if (last! < 0) return min(len, len + last!);
+    return min(len, last!);
   }
 }
diff --git a/pubspec.yaml b/pubspec.yaml
index 7435dcb..d60ee92 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,12 +1,13 @@
 name: json_path
-version: 0.2.0
+version: 0.3.0-nullsafety
 description: Implementation of JSONPath expressions like "$.store.book[2].price". Can read and set values in parsed JSON objects.
 homepage: "https://github.com/f3ath/jessie"
 
+environment:
+  sdk: '>=2.12.0-29 <3.0.0'
+
 dev_dependencies:
-  pedantic: ^1.9.0
-  test: ^1.9.0
-  test_coverage: ^0.4.3
+  pedantic: ^1.10.0-nullsafety
+  test: ^1.16.0-nullsafety
+  test_coverage: ^0.5.0
 
-environment:
-  sdk: ">=2.8.0 <3.0.0"