diff --git a/Sources/NIOWebSocket/NIOWebSocketClientUpgrader.swift b/Sources/NIOWebSocket/NIOWebSocketClientUpgrader.swift
index cc5fa16e33..e8f1b5ca47 100644
--- a/Sources/NIOWebSocket/NIOWebSocketClientUpgrader.swift
+++ b/Sources/NIOWebSocket/NIOWebSocketClientUpgrader.swift
@@ -199,7 +199,7 @@ private func _upgrade<UpgradeResult>(
             ByteToMessageHandler(WebSocketFrameDecoder(maxFrameSize: maxFrameSize))
         )
         if enableAutomaticErrorHandling {
-            try channel.pipeline.syncOperations.addHandler(WebSocketProtocolErrorHandler())
+            try channel.pipeline.syncOperations.addHandler(WebSocketProtocolErrorHandler(isServer: false))
         }
     }
     .flatMap {
diff --git a/Sources/NIOWebSocket/NIOWebSocketServerUpgrader.swift b/Sources/NIOWebSocket/NIOWebSocketServerUpgrader.swift
index 4f888a3e34..14e3c72612 100644
--- a/Sources/NIOWebSocket/NIOWebSocketServerUpgrader.swift
+++ b/Sources/NIOWebSocket/NIOWebSocketServerUpgrader.swift
@@ -62,11 +62,8 @@ extension HTTPHeaders {
 ///
 /// This upgrader assumes that the `HTTPServerUpgradeHandler` will appropriately mutate the pipeline to
 /// remove the HTTP `ChannelHandler`s.
-public final class NIOWebSocketServerUpgrader: HTTPServerProtocolUpgrader, @unchecked Sendable {
-    // This type *is* Sendable but we can't express that properly until Swift 5.7. In the meantime
-    // the conformance is `@unchecked`.
+public final class NIOWebSocketServerUpgrader: HTTPServerProtocolUpgrader, Sendable {
 
-    // FIXME: remove @unchecked when 5.7 is the minimum supported version.
     private typealias ShouldUpgrade = @Sendable (Channel, HTTPRequestHead) -> EventLoopFuture<HTTPHeaders?>
     private typealias UpgradePipelineHandler = @Sendable (Channel, HTTPRequestHead) -> EventLoopFuture<Void>
     /// RFC 6455 specs this as the required entry in the Upgrade header.
diff --git a/Sources/NIOWebSocket/WebSocketProtocolErrorHandler.swift b/Sources/NIOWebSocket/WebSocketProtocolErrorHandler.swift
index e367184b62..6c63126e9e 100644
--- a/Sources/NIOWebSocket/WebSocketProtocolErrorHandler.swift
+++ b/Sources/NIOWebSocket/WebSocketProtocolErrorHandler.swift
@@ -22,7 +22,20 @@ public final class WebSocketProtocolErrorHandler: ChannelInboundHandler {
     public typealias InboundIn = Never
     public typealias OutboundOut = WebSocketFrame
 
-    public init() {}
+    /// Indicate that this `ChannelHandeler` is used by a WebSocket server or client. Default is true.
+    private let isServer: Bool
+
+    public init() {
+        self.isServer = true
+    }
+
+    /// Initialize this `ChannelHandler` to be used by a WebSocket server or client.
+    ///
+    /// - Parameters:
+    ///     - isServer: indicate whether this `ChannelHandler` is used by a WebSocket server or client.
+    public init(isServer: Bool) {
+        self.isServer = isServer
+    }
 
     public func errorCaught(context: ChannelHandlerContext, error: Error) {
         let loopBoundContext = context.loopBound
@@ -32,6 +45,7 @@ public final class WebSocketProtocolErrorHandler: ChannelInboundHandler {
             let frame = WebSocketFrame(
                 fin: true,
                 opcode: .connectionClose,
+                maskKey: self.makeMaskingKey(),
                 data: data
             )
             context.writeAndFlush(Self.wrapOutboundOut(frame)).whenComplete { (_: Result<Void, Error>) in
@@ -44,6 +58,12 @@ public final class WebSocketProtocolErrorHandler: ChannelInboundHandler {
         // forward the error on to let others see it.
         context.fireErrorCaught(error)
     }
+
+    private func makeMaskingKey() -> WebSocketMaskingKey? {
+        // According to RFC 6455 Section 5, a client *must* mask all frames that it sends to the server.
+        // A server *must not* mask any frames that it sends to the client
+        self.isServer ? nil : .random()
+    }
 }
 
 @available(*, unavailable)
diff --git a/Tests/NIOWebSocketTests/WebSocketClientEndToEndTests.swift b/Tests/NIOWebSocketTests/WebSocketClientEndToEndTests.swift
index 71a86bdc1e..11e8f848e0 100644
--- a/Tests/NIOWebSocketTests/WebSocketClientEndToEndTests.swift
+++ b/Tests/NIOWebSocketTests/WebSocketClientEndToEndTests.swift
@@ -466,6 +466,35 @@ class WebSocketClientEndToEndTests: XCTestCase {
         // Close the pipeline.
         XCTAssertNoThrow(try clientChannel.close().wait())
     }
+
+    func testErrorHandlerMaskFrameForClient() throws {
+
+        let (clientChannel, _) = try self.runSuccessfulUpgrade()
+        let maskBitMask: UInt8 = 0x80
+
+        var data = clientChannel.allocator.buffer(capacity: 4)
+        // A fake frame header that claims that the length of the frame is 16385 bytes,
+        // larger than the frame max.
+        data.writeBytes([0x81, 0xFE, 0x40, 0x01])
+
+        XCTAssertThrowsError(try clientChannel.writeInbound(data)) { error in
+            XCTAssertEqual(.invalidFrameLength, error as? NIOWebSocketError)
+        }
+
+        clientChannel.embeddedEventLoop.run()
+        var buffer = try clientChannel.readAllOutboundBuffers()
+
+        guard let (_, secondByte) = buffer.readMultipleIntegers(as: (UInt8, UInt8).self) else {
+            XCTFail("Insufficient bytes from WebSocket frame")
+            return
+        }
+
+        let maskedBit = (secondByte & maskBitMask)
+        XCTAssertEqual(0x80, maskedBit)
+
+        XCTAssertNoThrow(!clientChannel.isActive)
+        XCTAssertTrue(try clientChannel.finish(acceptAlreadyClosed: true).isClean)
+    }
 }
 
 #if !canImport(Darwin) || swift(>=5.10)