diff --git a/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP1Connections.swift b/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP1Connections.swift
index 3cdf51869..63c0429f6 100644
--- a/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP1Connections.swift	
+++ b/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP1Connections.swift	
@@ -496,10 +496,16 @@ extension HTTPConnectionPool {
         /// - Parameter connectionID: The released connection's id.
         /// - Returns: An index and an IdleConnectionContext to determine the next action for the now idle connection.
         ///            Call ``leaseConnection(at:)`` or ``closeConnection(at:)`` with the supplied index after
-        ///            this. If you want to park the connection no further call is required.
-        mutating func releaseConnection(_ connectionID: Connection.ID) -> (Int, IdleConnectionContext) {
+        ///            this. If you want to park the connection no further call is required. `nil` if the
+        ///            connection is no longer known to the state machine.
+        mutating func releaseConnection(_ connectionID: Connection.ID) -> (Int, IdleConnectionContext)? {
             guard let index = self.connections.firstIndex(where: { $0.connectionID == connectionID }) else {
-                preconditionFailure("A connection that we don't know was released? Something is very wrong...")
+                // The connection's close/failure may have already been processed by the state
+                // machine (e.g. the peer closed the connection right as the response finished,
+                // racing with this release notification) — that path already removed the
+                // connection and found new work for any queued requests, so there's nothing left
+                // to do for this now-stale release.
+                return nil
             }
 
             self.connections[index].release()
diff --git a/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP1StateMachine.swift b/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP1StateMachine.swift
index 395064377..7ecb3a9d8 100644
--- a/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP1StateMachine.swift	
+++ b/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP1StateMachine.swift	
@@ -348,7 +348,12 @@ extension HTTPConnectionPool {
         }
 
         mutating func http1ConnectionReleased(_ connectionID: Connection.ID) -> Action {
-            let (index, context) = self.connections.releaseConnection(connectionID)
+            guard let (index, context) = self.connections.releaseConnection(connectionID) else {
+                // The connection was already closed/failed by the time this release was
+                // processed; that path already handled cleanup. See `http1ConnectionClosed`
+                // above for the same pattern.
+                return .none
+            }
             return .init(self.nextActionForIdleConnection(at: index, context: context))
         }
 
diff --git a/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP2StateMachine.swift b/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP2StateMachine.swift
index 67a07e6dd..4e9d7a064 100644
--- a/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP2StateMachine.swift	
+++ b/Sources/AsyncHTTPClient/ConnectionPool/State Machine/HTTPConnectionPool+HTTP2StateMachine.swift	
@@ -563,9 +563,14 @@ extension HTTPConnectionPool {
         }
 
         mutating func http1ConnectionReleased(_ connectionID: Connection.ID) -> Action {
-            // It is save to bang the http1Connections here. If we get this callback but we don't have
+            // It is safe to bang the http1Connections here. If we get this callback but we don't have
             // http1 connections something has gone terribly wrong.
-            let (index, _) = self.http1Connections!.releaseConnection(connectionID)
+            guard let (index, _) = self.http1Connections!.releaseConnection(connectionID) else {
+                // The connection was already closed/failed by the time this release was
+                // processed; that path already handled cleanup. See `http1ConnectionClosed`
+                // above for the same pattern.
+                return .none
+            }
             // Any http1 connection that becomes idle should be closed right away after the transition
             // to http2.
             let connection = self.http1Connections!.closeConnection(at: index)
diff --git a/Tests/AsyncHTTPClientTests/HTTPConnectionPool+HTTP1ConnectionsTest.swift b/Tests/AsyncHTTPClientTests/HTTPConnectionPool+HTTP1ConnectionsTest.swift
index 89f3bf7b5..01982fcd1 100644
--- a/Tests/AsyncHTTPClientTests/HTTPConnectionPool+HTTP1ConnectionsTest.swift
+++ b/Tests/AsyncHTTPClientTests/HTTPConnectionPool+HTTP1ConnectionsTest.swift
@@ -380,7 +380,9 @@ class HTTPConnectionPool_HTTP1ConnectionsTests: XCTestCase {
         XCTAssertEqual(connections.stats.connecting, 1)
         XCTAssertFalse(connections.isEmpty)
 
-        let (releaseIndex, _) = connections.releaseConnection(lease.id)
+        guard let (releaseIndex, _) = connections.releaseConnection(lease.id) else {
+            return XCTFail("Expected that the connection is remembered")
+        }
         XCTAssertEqual(connections.closeConnection(at: releaseIndex), lease)
         XCTAssertFalse(connections.isEmpty)