diff --git a/lib/src/parse_num.dart b/lib/src/parse_num.dart
index 27aeffe..ede562b 100644
--- a/lib/src/parse_num.dart
+++ b/lib/src/parse_num.dart
@@ -5,14 +5,15 @@ bool _isOtherNum(s) => _numAltRegex.hasMatch(s);
 bool isNum(String s) =>
     num.tryParse(s) != null || _isStandardNum(s) || _isOtherNum(s);
 
-num tryParseNum(String s) {
+num? tryParseNum(String s) {
   try {
     return num.parse(s);
-  } catch (FormatException) {
-    if (_isStandardNum(s))
+  } catch (e) {
+    if (_isStandardNum(s)) {
       return num.parse(s.replaceAll(',', ''));
-    else if (_isOtherNum(s))
+    } else if (_isOtherNum(s)) {
       return num.parse(s.replaceAll('.', '').replaceFirst(',', '.'));
+    }
     return null;
   }
 }
diff --git a/lib/src/rake_base.dart b/lib/src/rake_base.dart
index 826a855..ff3d83d 100644
--- a/lib/src/rake_base.dart
+++ b/lib/src/rake_base.dart
@@ -26,7 +26,6 @@ RegExp buildStopWordRegExp(List<String> stopWords) =>
 /// Split and collect phrases in sentences
 List<String> splitPhrases(List<String> sentences, RegExp stopWords,
     {int minChars = 0}) {
-  minChars = minChars ?? 0;
   List<String> phrases = [];
   for (final sentence in sentences) {
     phrases.addAll(sentence
@@ -48,38 +47,41 @@ Map<String, double> calculateWordScores(List<String> phrases) {
     final wordListDegree = wordListCount - 1;
     for (final word in wordList) {
       wordFrequency.putIfAbsent(word, () => 0);
-      wordFrequency[word] += 1;
+      wordFrequency[word] = wordFrequency[word]! + 1;
       wordDegree.putIfAbsent(word, () => 0);
-      wordDegree[word] += wordListDegree;
+      wordDegree[word] = wordDegree[word]! + wordListDegree;
     }
   }
-  for (final word in wordFrequency.keys)
-    wordDegree[word] += wordFrequency[word];
+  for (final word in wordFrequency.keys) {
+    wordDegree[word] = wordDegree[word]! + wordFrequency[word]!;
+  }
 
   Map<String, double> wordScore = Map();
   for (final word in wordFrequency.keys) {
     wordScore.putIfAbsent(word, () => 0);
-    wordScore[word] = wordDegree[word] / wordFrequency[word];
+    wordScore[word] = wordDegree[word]! / wordFrequency[word]!;
   }
   return wordScore;
 }
 
 Map<String, double> calculateKeywordScores(
     List<String> phrases, Map<String, double> wordScore,
-    {int minFrequency}) {
+    {int? minFrequency}) {
   Map<String, double> keywordCandidates = Map();
-  Map<String, int> frequencies;
+  late Map<String, int> frequencies;
   if (minFrequency != null) {
     frequencies = Map();
     phrases.forEach((phrase) =>
         frequencies.update(phrase, (v) => v + 1, ifAbsent: () => 1));
   }
   for (final phrase in phrases) {
-    if (minFrequency != null && frequencies[phrase] < minFrequency) continue;
+    if (minFrequency != null && frequencies[phrase]! < minFrequency) continue;
     keywordCandidates.putIfAbsent(phrase, () => 0);
     final words = splitWords(phrase);
     double candidateScore = 0;
-    for (final word in words) candidateScore += wordScore[word];
+    for (final word in words) {
+      candidateScore += wordScore[word]!;
+    }
     keywordCandidates[phrase] = candidateScore;
   }
   return keywordCandidates;
@@ -93,22 +95,22 @@ class Rake {
   final RegExp _stopWordPattern;
 
   /// Extract keywords and sores found in a text
-  Map<String, double> run(String text, {int minChars, int minFrequency}) {
+  Map<String, double> run(String text, {int? minChars, int? minFrequency}) {
     final List<String> sentences = splitSentences(text);
     final List<String> phrases =
-        splitPhrases(sentences, _stopWordPattern, minChars: minChars);
+        splitPhrases(sentences, _stopWordPattern, minChars: (minChars ?? 0));
     final Map<String, double> wordScores = calculateWordScores(phrases);
     return calculateKeywordScores(phrases, wordScores,
         minFrequency: minFrequency);
   }
 
   /// Extract a list of keywords, ordered by score
-  List<String> rank(String text, {int minChars, int minFrequency}) {
+  List<String> rank(String text, {int? minChars, int? minFrequency}) {
     final keywordScores =
         run(text, minFrequency: minFrequency, minChars: minChars);
     final keywords = keywordScores.keys.toList();
-    keywords.sort(
-        (a, b) => a == b ? 0 : (keywordScores[a] > keywordScores[b] ? -1 : 1));
+    keywords.sort((a, b) =>
+        a == b ? 0 : (keywordScores[a]! > keywordScores[b]! ? -1 : 1));
     return keywords;
   }
 }
diff --git a/pubspec.yaml b/pubspec.yaml
index 3538e21..1944d38 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,8 +5,8 @@ homepage: https://github.com/mathisgerdes/dart-rake
 author: Mathis Gerdes <mathisgerdes@gmail.com>
 
 environment:
-  sdk: '>=2.1.0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dev_dependencies:
-  pedantic: ^1.0.0
-  test: ^1.0.0
+  pedantic: ^1.11.0
+  test: ^1.17.3