diff --git a/protobuf/lib/src/protobuf/builder_info.dart b/protobuf/lib/src/protobuf/builder_info.dart
index 5d1019e7..28997e4c 100644
--- a/protobuf/lib/src/protobuf/builder_info.dart
+++ b/protobuf/lib/src/protobuf/builder_info.dart
@@ -120,7 +120,7 @@ class BuilderInfo {
     int tagNumber,
     String name,
     int fieldType,
-    CheckFunc<T> check,
+    CheckFunc<T>? check,
     CreateBuilderFunc? subBuilder,
     ValueOfFunc? valueOf,
     List<ProtobufEnum>? enumValues, {
diff --git a/protobuf/lib/src/protobuf/extension.dart b/protobuf/lib/src/protobuf/extension.dart
index dc37b91f..1af1529a 100644
--- a/protobuf/lib/src/protobuf/extension.dart
+++ b/protobuf/lib/src/protobuf/extension.dart
@@ -35,7 +35,7 @@ class Extension<T> extends FieldInfo<T> {
     String name,
     int tagNumber,
     int fieldType, {
-    required CheckFunc<T> check,
+    required CheckFunc<T>? check,
     CreateBuilderFunc? subBuilder,
     ValueOfFunc? valueOf,
     List<ProtobufEnum>? enumValues,
diff --git a/protobuf/lib/src/protobuf/field_error.dart b/protobuf/lib/src/protobuf/field_error.dart
index c6d7e40c..a9116023 100644
--- a/protobuf/lib/src/protobuf/field_error.dart
+++ b/protobuf/lib/src/protobuf/field_error.dart
@@ -72,8 +72,8 @@ String? _getFieldError(int fieldType, var value) {
 /// unsigned 32 bit ints where there also is a range check.
 ///
 /// @nodoc
-CheckFunc getCheckFunction(int fieldType) {
-  switch (fieldType & ~0x7) {
+CheckFunc? getCheckFunction(int fieldType) {
+  switch (PbFieldType.baseType(fieldType)) {
     case PbFieldType.BOOL_BIT:
     case PbFieldType.BYTES_BIT:
     case PbFieldType.STRING_BIT:
@@ -89,7 +89,7 @@ CheckFunc getCheckFunction(int fieldType) {
       // We always use the full range of the same Dart type.
       // It's up to the caller to treat the Int64 as signed or unsigned.
       // See: https://github.com/google/protobuf.dart/issues/44
-      return checkNotNull;
+      return null;
 
     case PbFieldType.FLOAT_BIT:
       return _checkFloat;
diff --git a/protobuf/lib/src/protobuf/field_info.dart b/protobuf/lib/src/protobuf/field_info.dart
index cf5b28cc..775b3c0f 100644
--- a/protobuf/lib/src/protobuf/field_info.dart
+++ b/protobuf/lib/src/protobuf/field_info.dart
@@ -138,7 +138,7 @@ class FieldInfo<T> {
     this.tagNumber,
     this.index,
     this.type,
-    CheckFunc<T> this.check,
+    this.check,
     this.subBuilder, {
     this.valueOf,
     this.enumValues,
@@ -232,13 +232,13 @@ class FieldInfo<T> {
   /// Creates a repeated field.
   PbList<T> _createRepeatedField() {
     assert(isRepeated);
-    return PbList<T>(check: check!);
+    return PbList<T>(check: check);
   }
 
   /// Same as above, but allow a tighter typed [PbList] to be created.
   PbList<S> _createRepeatedFieldWithType<S extends T>() {
     assert(isRepeated);
-    return PbList<S>(check: check!);
+    return PbList<S>(check: check);
   }
 
   /// Convenience method to thread this FieldInfo's reified type parameter to
diff --git a/protobuf/lib/src/protobuf/pb_list.dart b/protobuf/lib/src/protobuf/pb_list.dart
index 4736f6c3..1c2c8ce4 100644
--- a/protobuf/lib/src/protobuf/pb_list.dart
+++ b/protobuf/lib/src/protobuf/pb_list.dart
@@ -29,15 +29,13 @@ class PbList<E> extends ListBase<E> {
   /// We can't use `const []` as it makes the `_wrappedList` field polymorphic.
   static final _emptyList = <Never>[];
 
-  final CheckFunc<E> _check;
+  final CheckFunc<E>? _check;
 
   bool _isReadOnly = false;
 
   bool get isFrozen => _isReadOnly;
 
-  PbList({CheckFunc<E> check = checkNotNull})
-    : _wrappedList = <E>[],
-      _check = check;
+  PbList({CheckFunc<E>? check}) : _wrappedList = <E>[], _check = check;
 
   PbList.unmodifiable()
     : _wrappedList = _emptyList,
@@ -52,7 +50,9 @@ class PbList<E> extends ListBase<E> {
   @pragma('dart2js:never-inline')
   void add(E element) {
     _checkModifiable('add');
-    _check(element);
+    if (_check != null) {
+      _check(element);
+    }
     _wrappedList.add(element);
   }
 
@@ -67,7 +67,9 @@ class PbList<E> extends ListBase<E> {
   @pragma('dart2js:never-inline')
   void addAll(Iterable<E> iterable) {
     _checkModifiable('addAll');
-    iterable.forEach(_check);
+    if (_check != null) {
+      iterable.forEach(_check);
+    }
     _wrappedList.addAll(iterable);
   }
 
@@ -96,21 +98,27 @@ class PbList<E> extends ListBase<E> {
   @override
   void insert(int index, E element) {
     _checkModifiable('insert');
-    _check(element);
+    if (_check != null) {
+      _check(element);
+    }
     _wrappedList.insert(index, element);
   }
 
   @override
   void insertAll(int index, Iterable<E> iterable) {
     _checkModifiable('insertAll');
-    iterable.forEach(_check);
+    if (_check != null) {
+      iterable.forEach(_check);
+    }
     _wrappedList.insertAll(index, iterable);
   }
 
   @override
   void setAll(int index, Iterable<E> iterable) {
     _checkModifiable('setAll');
-    iterable.forEach(_check);
+    if (_check != null) {
+      iterable.forEach(_check);
+    }
     _wrappedList.setAll(index, iterable);
   }
 
@@ -149,7 +157,9 @@ class PbList<E> extends ListBase<E> {
     _checkModifiable('setRange');
     // NOTE: In case `take()` returns less than `end - start` elements, the
     // _wrappedList will fail with a `StateError`.
-    iterable.skip(skipCount).take(end - start).forEach(_check);
+    if (_check != null) {
+      iterable.skip(skipCount).take(end - start).forEach(_check);
+    }
     _wrappedList.setRange(start, end, iterable, skipCount);
   }
 
@@ -162,7 +172,9 @@ class PbList<E> extends ListBase<E> {
   @override
   void fillRange(int start, int end, [E? fill]) {
     _checkModifiable('fillRange');
-    _check(fill);
+    if (_check != null) {
+      _check(fill);
+    }
     _wrappedList.fillRange(start, end, fill);
   }
 
@@ -170,7 +182,9 @@ class PbList<E> extends ListBase<E> {
   void replaceRange(int start, int end, Iterable<E> newContents) {
     _checkModifiable('replaceRange');
     final values = newContents.toList();
-    newContents.forEach(_check);
+    if (_check != null) {
+      newContents.forEach(_check);
+    }
     _wrappedList.replaceRange(start, end, values);
   }
 
@@ -202,7 +216,9 @@ class PbList<E> extends ListBase<E> {
   @override
   void operator []=(int index, E value) {
     _checkModifiable('set element');
-    _check(value);
+    if (_check != null) {
+      _check(value);
+    }
     _wrappedList[index] = value;
   }