diff --git a/lib/http_connection.dart b/lib/http_connection.dart
index b2a70ff..80a9308 100644
--- a/lib/http_connection.dart
+++ b/lib/http_connection.dart
@@ -133,7 +133,7 @@ class TransportSendQueue {
 
   Future<void> stop() {
     _executing = false;
-    _sendBufferedData.complete();
+    if (!_sendBufferedData.isCompleted) _sendBufferedData.complete();
     return _sendLoopPromise;
   }
 
@@ -149,7 +149,7 @@ class TransportSendQueue {
     }
 
     _buffer.add(data);
-    _sendBufferedData.complete();
+    if (!_sendBufferedData.isCompleted) _sendBufferedData.complete();
   }
 
   Future<void> _sendLoop() async {
@@ -158,7 +158,9 @@ class TransportSendQueue {
 
       if (!_executing) {
         if (_transportResult != null) {
-          _transportResult.completeError("Connection stopped.");
+          if (!_transportResult.isCompleted) {
+            _transportResult.completeError('Connection stopped.');
+          }
         }
 
         break;
@@ -177,11 +179,9 @@ class TransportSendQueue {
 
       try {
         await this.transport.send(data);
-        if (!transportResult.isCompleted) {
-          transportResult.complete();
-        }
+        if (!transportResult.isCompleted) transportResult.complete();
       } catch (error) {
-        transportResult.completeError(error);
+        if (!transportResult.isCompleted) transportResult.completeError(error);
       }
     }
   }
@@ -648,7 +648,7 @@ class HttpConnection implements IConnection {
     if (_connectionState == ConnectionState.Disconnecting) {
       // A call to stop() induced this call to stopConnection and needs to be completed.
       // Any stop() awaiters will be scheduled to continue after the onclose callback fires.
-      _stopPromiseCompleter.complete();
+      if (!_stopPromiseCompleter.isCompleted) _stopPromiseCompleter.complete();
     }
 
     if (error != null) {
diff --git a/lib/hub_connection.dart b/lib/hub_connection.dart
index f8f648b..3bc6f05 100644
--- a/lib/hub_connection.dart
+++ b/lib/hub_connection.dart
@@ -397,25 +397,31 @@ class HubConnection {
     _callbacks[invocationDescriptor.invocationId] =
         (HubMessageBase invocationEvent, Exception error) {
       if (error != null) {
-        completer.completeError(error);
+        if (!completer.isCompleted) completer.completeError(error);
         return;
       } else if (invocationEvent != null) {
         if (invocationEvent is CompletionMessage) {
           if (invocationEvent.error != null) {
-            completer.completeError(new GeneralError(invocationEvent.error));
+            if (!completer.isCompleted) {
+              completer.completeError(new GeneralError(invocationEvent.error));
+            }
           } else {
-            completer.complete(invocationEvent.result);
+            if (!completer.isCompleted) {
+              completer.complete(invocationEvent.result);
+            }
           }
         } else {
-          completer.completeError(new GeneralError(
-              "Unexpected message type: ${invocationEvent.type}"));
+          if (!completer.isCompleted) {
+            completer.completeError(new GeneralError(
+                "Unexpected message type: ${invocationEvent.type}"));
+          }
         }
       }
     };
 
     final promiseQueue =
         _sendWithProtocol(invocationDescriptor).catchError((e) {
-      completer.completeError(e);
+      if (!completer.isCompleted) completer.completeError(e);
       // invocationId will always have a value for a non-blocking invocation
       _callbacks.remove(invocationDescriptor.invocationId);
     });
@@ -586,7 +592,9 @@ class HubConnection {
 
       final error = GeneralError(message);
 
-      _handshakeCompleter?.completeError(error);
+      if (!_handshakeCompleter.isCompleted) {
+        _handshakeCompleter?.completeError(error);
+      }
       _handshakeCompleter = null;
       throw error;
     }
@@ -597,14 +605,16 @@ class HubConnection {
 
       final error = GeneralError(message);
 
-      _handshakeCompleter?.completeError(error);
+      if (!_handshakeCompleter.isCompleted) {
+        _handshakeCompleter?.completeError(error);
+      }
       _handshakeCompleter = null;
       throw error;
     } else {
       _logger?.finer("Server handshake complete.");
     }
 
-    _handshakeCompleter?.complete();
+    if (!_handshakeCompleter.isCompleted) _handshakeCompleter?.complete();
     _handshakeCompleter = null;
     return handshakeResult.remainingData;
   }
@@ -677,7 +687,7 @@ class HubConnection {
     // If the handshake is in progress, start will be waiting for the handshake promise, so we complete it.
     // If it has already completed, this should just noop.
     if (_handshakeCompleter != null) {
-      _handshakeCompleter.complete();
+      if (!_handshakeCompleter.isCompleted) _handshakeCompleter.complete();
     }
 
     _cancelCallbacksWithError(error ??
diff --git a/lib/web_socket_transport.dart b/lib/web_socket_transport.dart
index 7d3f751..303391b 100644
--- a/lib/web_socket_transport.dart
+++ b/lib/web_socket_transport.dart
@@ -52,7 +52,7 @@ class WebSocketTransport implements ITransport {
     _logger?.finest("WebSocket try connecting to '$url'.");
     _webSocket = WebSocketChannel.connect(Uri.parse(url));
     opened = true;
-    websocketCompleter.complete();
+    if (!websocketCompleter.isCompleted) websocketCompleter.complete();
     _logger?.info("WebSocket connected to '$url'.");
     _webSocketListenSub = _webSocket.stream.listen(
       // onData
@@ -77,7 +77,9 @@ class WebSocketTransport implements ITransport {
       // onError
       onError: (Object error) {
         var e = error != null ? error : "Unknown websocket error";
-        websocketCompleter.completeError(e);
+        if (!websocketCompleter.isCompleted) {
+          websocketCompleter.completeError(e);
+        }
       },
 
       // onDone
@@ -89,8 +91,10 @@ class WebSocketTransport implements ITransport {
             onClose();
           }
         } else {
-          websocketCompleter
-              .completeError("There was an error with the transport.");
+          if (!websocketCompleter.isCompleted) {
+            websocketCompleter
+                .completeError("There was an error with the transport.");
+          }
         }
       },
     );
diff --git a/lib/web_supporting_http_client.dart b/lib/web_supporting_http_client.dart
index 02e2124..e382984 100644
--- a/lib/web_supporting_http_client.dart
+++ b/lib/web_supporting_http_client.dart
@@ -48,8 +48,9 @@ class WebSupportingHttpClient extends SignalRHttpClient {
       final abortFuture = Future<void>(() {
         final completer = Completer<void>();
         if (request.abortSignal != null) {
-          request.abortSignal.onabort =
-              () => completer.completeError(AbortError());
+          request.abortSignal.onabort = () {
+            if (!completer.isCompleted) completer.completeError(AbortError());
+          };
         }
         return completer.future;
       });