diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3de6f7bc..33f414f2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
-#### 3.2.0
+#### 3.2.0-dev
 
+  * Explicitly disallow a `nullable` type argument for `Optional`.
   * Expose `TreeIterator` for iterating `TreeSet` instead of using
     `BidirectionalIterator`.
   * Require Dart 2.17
diff --git a/analysis_options.yaml b/analysis_options.yaml
index deee90df..2dc1dc35 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -1,11 +1,6 @@
 # https://dart.dev/guides/language/analysis-options
 include: package:lints/recommended.yaml
 
-analyzer:
-  errors:
-    # TODO(kevmoo): see https://github.com/dart-lang/linter/issues/3844
-    null_check_on_nullable_type_parameter: ignore
-
 linter:
   rules:
     - always_declare_return_types
diff --git a/lib/src/collection/bimap.dart b/lib/src/collection/bimap.dart
index cc41456d..6dd45a3e 100644
--- a/lib/src/collection/bimap.dart
+++ b/lib/src/collection/bimap.dart
@@ -151,7 +151,7 @@ class HashBiMap<K, V> implements BiMap<K, V> {
   @override
   void updateAll(V Function(K key, V value) update) {
     for (final key in keys) {
-      _add(key, update(key, _map[key]!), true);
+      _add(key, update(key, _map[key] as V), true);
     }
   }
 
diff --git a/lib/src/collection/lru_map.dart b/lib/src/collection/lru_map.dart
index fabbcf72..b0a927eb 100644
--- a/lib/src/collection/lru_map.dart
+++ b/lib/src/collection/lru_map.dart
@@ -290,7 +290,7 @@ class LinkedLruHashMap<K, V> implements LruMap<K, V> {
   V update(K key, V Function(V value) update, {V Function()? ifAbsent}) {
     V newValue;
     if (containsKey(key)) {
-      newValue = update(this[key]!);
+      newValue = update(this[key] as V);
     } else {
       if (ifAbsent == null) {
         throw ArgumentError.value(key, 'key', 'Key not in map');
diff --git a/lib/src/collection/treeset.dart b/lib/src/collection/treeset.dart
index c42cc704..743f9e0c 100644
--- a/lib/src/collection/treeset.dart
+++ b/lib/src/collection/treeset.dart
@@ -899,7 +899,9 @@ class TreeIterator<V>
       {this.reversed = false, this.inclusive = true, V? anchorObject})
       : _anchorObject = anchorObject,
         _modCountGuard = tree._modCount {
-    if (_anchorObject == null || tree.isEmpty) {
+    final anchor = _anchorObject;
+
+    if (anchor == null || tree.isEmpty) {
       // If the anchor is far left or right, we're just a normal iterator.
       _state = reversed ? _right : _left;
       _moveNext = reversed ? _movePreviousNormal : _moveNextNormal;
@@ -911,28 +913,26 @@ class TreeIterator<V>
     // Else we've got an anchor we have to worry about initializing from.
     // This isn't known till the caller actually performs a previous/next.
     _moveNext = () {
-      _current = tree._searchNearest(_anchorObject,
+      _current = tree._searchNearest(anchor,
           option: reversed ? TreeSearch.LESS_THAN : TreeSearch.GREATER_THAN);
       _moveNext = reversed ? _movePreviousNormal : _moveNextNormal;
       _movePrevious = reversed ? _moveNextNormal : _movePreviousNormal;
       if (_current == null) {
         _state = reversed ? _left : _right;
-      } else if (tree.comparator(_current!.object, _anchorObject!) == 0 &&
-          !inclusive) {
+      } else if (tree.comparator(_current!.object, anchor) == 0 && !inclusive) {
         _moveNext();
       }
       return _state == _walk;
     };
 
     _movePrevious = () {
-      _current = tree._searchNearest(_anchorObject,
+      _current = tree._searchNearest(anchor,
           option: reversed ? TreeSearch.GREATER_THAN : TreeSearch.LESS_THAN);
       _moveNext = reversed ? _movePreviousNormal : _moveNextNormal;
       _movePrevious = reversed ? _moveNextNormal : _movePreviousNormal;
       if (_current == null) {
         _state = reversed ? _right : _left;
-      } else if (tree.comparator(_current!.object, _anchorObject!) == 0 &&
-          !inclusive) {
+      } else if (tree.comparator(_current!.object, anchor) == 0 && !inclusive) {
         _movePrevious();
       }
       return _state == _walk;
diff --git a/lib/src/core/optional.dart b/lib/src/core/optional.dart
index ed8ec894..42ab9e51 100644
--- a/lib/src/core/optional.dart
+++ b/lib/src/core/optional.dart
@@ -23,7 +23,7 @@ import 'dart:collection';
 /// With the introduction of non-null by default in Dart SDK 2.12, developers
 /// should avoid adding more uses of this type. Existing users should migrate
 /// away from the `Optional` type to types marked nullable: `T?`.
-class Optional<T> extends IterableBase<T> {
+class Optional<T extends Object> extends IterableBase<T> {
   /// Constructs an empty Optional.
   const Optional.absent() : _value = null;
 
diff --git a/lib/src/iterables/generating_iterable.dart b/lib/src/iterables/generating_iterable.dart
index 8facce80..9796b1b9 100644
--- a/lib/src/iterables/generating_iterable.dart
+++ b/lib/src/iterables/generating_iterable.dart
@@ -67,9 +67,10 @@ class _GeneratingIterator<T> implements Iterator<T> {
 
   @override
   bool moveNext() {
-    if (object == null) return false;
+    final obj = object;
+    if (obj == null) return false;
     if (started) {
-      object = next(object!);
+      object = next(obj);
     } else {
       started = true;
     }