diff --git a/CHANGELOG.md b/CHANGELOG.md
index 142629a..0391759 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.5.2
+
+- Fix releasing of locks when closing `SharedMutex``.
+
 ## 0.5.1
 
 - Fix `watch` when called with query parameters.
diff --git a/lib/src/mutex.dart b/lib/src/mutex.dart
index 578e1d4..5725554 100644
--- a/lib/src/mutex.dart
+++ b/lib/src/mutex.dart
@@ -126,6 +126,7 @@ class SerializedMutex {
 /// Uses a [SendPort] to communicate with the source mutex.
 class SharedMutex implements Mutex {
   final ChildPortClient client;
+  bool closed = false;
 
   SharedMutex._(this.client);
 
@@ -135,6 +136,9 @@ class SharedMutex implements Mutex {
       throw LockError('Recursive lock is not allowed');
     }
     return runZoned(() async {
+      if (closed) {
+        throw const ClosedException();
+      }
       await _acquire(timeout: timeout);
       try {
         final T result = await callback();
@@ -174,7 +178,20 @@ class SharedMutex implements Mutex {
   }
 
   @override
+
+  /// Wait for existing locks to be released, then close this SharedMutex
+  /// and prevent further locks from being taken out.
   Future<void> close() async {
+    if (closed) {
+      return;
+    }
+    closed = true;
+    // Wait for any existing locks to complete, then prevent any further locks from being taken out.
+    await _acquire();
+    client.fire(const _CloseMessage());
+    // Close client immediately after _unlock(),
+    // so that we're sure no further locks are acquired.
+    // This also cancels any lock request in process.
     client.close();
   }
 }
@@ -184,6 +201,7 @@ class _SharedMutexServer {
   Completer? unlock;
   late final SerializedMutex serialized;
   final Mutex mutex;
+  bool closed = false;
 
   late final PortServer server;
 
@@ -198,6 +216,11 @@ class _SharedMutexServer {
     if (arg is _AcquireMessage) {
       var lock = Completer.sync();
       mutex.lock(() async {
+        if (closed) {
+          // The client will error already - we just need to ensure
+          // we don't take out another lock.
+          return;
+        }
         assert(unlock == null);
         unlock = Completer.sync();
         lock.complete();
@@ -208,6 +231,10 @@ class _SharedMutexServer {
     } else if (arg is _UnlockMessage) {
       assert(unlock != null);
       unlock!.complete();
+    } else if (arg is _CloseMessage) {
+      // Unlock and close (from client side)
+      closed = true;
+      unlock?.complete();
     }
   }
 
@@ -224,6 +251,11 @@ class _UnlockMessage {
   const _UnlockMessage();
 }
 
+/// Unlock and close
+class _CloseMessage {
+  const _CloseMessage();
+}
+
 class LockError extends Error {
   final String message;
 
diff --git a/lib/src/port_channel.dart b/lib/src/port_channel.dart
index 717c3d0..3d1ac42 100644
--- a/lib/src/port_channel.dart
+++ b/lib/src/port_channel.dart
@@ -125,6 +125,7 @@ class ChildPortClient implements PortClient {
   final SendPort sendPort;
   final ReceivePort receivePort = ReceivePort();
   int _nextId = 1;
+  bool closed = false;
 
   final Map<int, Completer<Object?>> handlers = HashMap();
 
@@ -144,6 +145,9 @@ class ChildPortClient implements PortClient {
 
   @override
   Future<T> post<T>(Object message) async {
+    if (closed) {
+      throw const ClosedException();
+    }
     var completer = Completer<T>.sync();
     var id = _nextId++;
     handlers[id] = completer;
@@ -153,11 +157,14 @@ class ChildPortClient implements PortClient {
 
   @override
   void fire(Object message) {
+    if (closed) {
+      throw ClosedException();
+    }
     sendPort.send(_FireMessage(message));
   }
 
   void _cancelAll(Object error) {
-    var handlers = this.handlers;
+    var handlers = HashMap<int, Completer<Object?>>.from(this.handlers);
     this.handlers.clear();
     for (var message in handlers.values) {
       message.completeError(error);
@@ -165,6 +172,7 @@ class ChildPortClient implements PortClient {
   }
 
   void close() {
+    closed = true;
     _cancelAll(const ClosedException());
     receivePort.close();
   }
diff --git a/pubspec.yaml b/pubspec.yaml
index 6d81a86..1c9d84f 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,6 +1,6 @@
 name: sqlite_async
 description: High-performance asynchronous interface for SQLite on Dart and Flutter.
-version: 0.5.1
+version: 0.5.2
 repository: https://github.com/journeyapps/sqlite_async.dart
 environment:
   sdk: '>=2.19.1 <4.0.0'
diff --git a/test/mutex_test.dart b/test/mutex_test.dart
new file mode 100644
index 0000000..0eb90e1
--- /dev/null
+++ b/test/mutex_test.dart
@@ -0,0 +1,47 @@
+import 'dart:isolate';
+
+import 'package:sqlite_async/mutex.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Mutex Tests', () {
+    test('Closing', () async {
+      // Test that locks are properly released when calling SharedMutex.close()
+      // in in Isolate.
+      // A timeout in this test indicates a likely error.
+      for (var i = 0; i < 50; i++) {
+        final mutex = SimpleMutex();
+        final serialized = mutex.shared;
+
+        final result = await Isolate.run(() async {
+          return _lockInIsolate(serialized);
+        });
+
+        await mutex.lock(() async {});
+
+        expect(result, equals(5));
+      }
+    });
+  }, timeout: const Timeout(Duration(milliseconds: 5000)));
+}
+
+Future<Object> _lockInIsolate(
+  SerializedMutex smutex,
+) async {
+  final mutex = smutex.open();
+  // Start a "thread" that repeatedly takes a lock
+  _infiniteLock(mutex).ignore();
+  await Future.delayed(const Duration(milliseconds: 10));
+  // Then close the mutex while the above loop is running.
+  await mutex.close();
+
+  return 5;
+}
+
+Future<void> _infiniteLock(SharedMutex mutex) async {
+  while (true) {
+    await mutex.lock(() async {
+      await Future.delayed(const Duration(milliseconds: 1));
+    });
+  }
+}