diff --git a/mirai-core/src/commonMain/kotlin/network/handler/NetworkHandler.kt b/mirai-core/src/commonMain/kotlin/network/handler/NetworkHandler.kt
index 2423988ce..088a2b2d1 100644
--- a/mirai-core/src/commonMain/kotlin/network/handler/NetworkHandler.kt
+++ b/mirai-core/src/commonMain/kotlin/network/handler/NetworkHandler.kt
@@ -148,6 +148,8 @@ internal interface NetworkHandler : CoroutineScope {
 
     /**
      * Closes this handler gracefully (i.e. asynchronously).
+     *
+     * After invocation of [close], [state] will always be [State.CLOSED].
      */
     fun close(cause: Throwable?)
 
diff --git a/mirai-core/src/commonMain/kotlin/network/handler/NetworkHandlerSupport.kt b/mirai-core/src/commonMain/kotlin/network/handler/NetworkHandlerSupport.kt
index 0dd0028f2..ec3470244 100644
--- a/mirai-core/src/commonMain/kotlin/network/handler/NetworkHandlerSupport.kt
+++ b/mirai-core/src/commonMain/kotlin/network/handler/NetworkHandlerSupport.kt
@@ -47,11 +47,6 @@ internal abstract class NetworkHandlerSupport(
     }
 
     override fun close(cause: Throwable?) {
-//        if (cause == null) {
-//            logger.info { "NetworkHandler '$this' closed" }
-//        } else {
-//            logger.info { "NetworkHandler '$this' closed: $cause" }
-//        }
         if (coroutineContext.job.isActive) {
             coroutineContext.job.cancel("NetworkHandler closed", cause)
         }
@@ -202,6 +197,7 @@ internal abstract class NetworkHandlerSupport(
     protected data class StateSwitchingException(
         val old: BaseStateImpl,
         val new: BaseStateImpl,
+        override val cause: Throwable? = new.getCause(), // so it can be unwrapped
     ) : CancellationException("State is switched from $old to $new")
 
     /**
diff --git a/mirai-core/src/commonMain/kotlin/network/impl/netty/NettyNetworkHandler.kt b/mirai-core/src/commonMain/kotlin/network/impl/netty/NettyNetworkHandler.kt
index 1d92ddcf5..2c9cffc85 100644
--- a/mirai-core/src/commonMain/kotlin/network/impl/netty/NettyNetworkHandler.kt
+++ b/mirai-core/src/commonMain/kotlin/network/impl/netty/NettyNetworkHandler.kt
@@ -195,15 +195,14 @@ internal open class NettyNetworkHandler(
     ///////////////////////////////////////////////////////////////////////////
 
     override fun close(cause: Throwable?) {
-        if (state == State.CLOSED) return // already
-        super.close(cause)
-        // when coroutine scope completed, state is already set to CLOSED,
-        // see the following `init` block.
+        if (state == State.CLOSED) return // quick check if already closed
+        if (setState { StateClosed(cause) } == null) return // atomic check
+        super.close(cause) // cancel coroutine scope
     }
 
     init {
         coroutineContext.job.invokeOnCompletion { e ->
-            setState { StateClosed(e?.unwrapCancellationException()) }
+            close(e?.unwrapCancellationException())
         }
     }