             sendOperationID = send(pdu) { [unowned self] in self.readByGroupType($0, operation: operation) }
         }
         
+        /// immediately call completion with error
         guard sendOperationID != nil else {
             completion(.error(.queueFull))
+            return
         }
     }
     

        
    }
    
    @inline(__always)
    private func send <Request: ATTProtocolDataUnit, Response: ATTProtocolDataUnit> (_ request: Request, response: @escaping (ATTResponse<Response>) -> ()) -> UInt? {
        
        log?("Request: \(request)")

                                               attributeType: operation.serviceType.rawValue,
                                               attributeValue: operation.uuid?.littleEndian ?? [])
                
                guard let _ = send(pdu, response: { [unowned self] in self.findByType($0, operation: operation) })
                    else { operation.completion(.error(.queueFull)); return }
                
                
            }
