struct.error: bad char in struct format

I have a problem with a little server-client assignment in python 2.7.

The client can send 5 types of requests to the server:

1. get the server's ip
2. get contents of a directory on the server
3. run cmd command on the server and get the output
4. open a calculator on the server
5. disconnect

basically, this is the error I get:

line 19, in server    data_size = calcsize(client_structs) - 3struct.error: bad char in struct format

Would appreciate explanation about this error + how to solve it.

server code:

__author__ = 'eyal'from struct import pack, unpack, calcsizeimport socketfrom os import listdirfrom subprocess import check_output, calldef server():    ser_soc = socket.socket()    ser_soc.bind(("0.0.0.0", 8080))    ser_soc.listen(1)    while True:        accept_flag = raw_input("Would you like to wait for a client? (y/n) ")        if accept_flag == "y":            client_soc, client_address = ser_soc.accept()            while True:                client_structs = client_soc.recv(1024)                data_size = calcsize(client_structs) - 3                data_str = 'c' * data_size                unpacked_data = unpack("BH"+ data_str, client_structs)                if unpacked_data[0] == 1:                    ip = socket.gethostbyname(socket.gethostname())                    ip_data = 'c' * len(ip)                    to_send = pack("BH"+ str(len(ip)) + ip_data, unpacked_data[0], len(ip), ip)                elif unpacked_data[0] == 2:                    content = listdir(str(unpacked_data[2]))                    content_str = "\r\n".join(content)                    content_data = 'c' * len(content_str)                    to_send = pack("BH"+ str(len(content_str)) + content_data, unpacked_data[0],                                   len(content_str), content_str)                elif unpacked_data[0] == 3:                    command = str(unpacked_data[2:]).split()                    output = check_output(command)                    message_data = 'c' * len(output)                    to_send = pack("BH"+ message_data, unpacked_data[0], len(output), output)                elif unpacked_data[0] == 4:                    call("gnome-calculator")                    msg_data = 'c' * len("The calculator is open.")                    to_send = pack("BH"+ msg_data, unpacked_data[0], len("The calculator is open."),"The calculator is open.")                elif unpacked_data[0] == 5:                    client_soc.close()                    break                else:                    to_send = pack("BH"+'c' * len("invalid message type, try again"),                                   unpacked_data[0], len("invalid message type, try again"),"invalid message type, try again")                if unpacked_data[0] != 5:                    client_soc.send(to_send)        else:            break    ser_soc.close()def main():    server()if __name__ == "__main__":    main()

client code:

__author__ = 'eyal'from struct import pack, unpack, calcsizeimport socketdef client():    my_soc = socket.socket()    my_soc.connect(("127.0.0.1", 8080))    while True:        send_flag = raw_input("Would you like to send the server a request? (y/n) ")        if send_flag == "y":            msg_code = input("What type of request would you like to send?\n""1. Get the server's IP address.\n""2. Get content of a directory on the server.\n""3. Run a terminal command on the server and get the output.\n""4. Open a calculator on the server.\n""5. Disconnect from the server.\n""Your choice: ")            if msg_code == 1 or msg_code == 4 or msg_code == 5:                to_send = pack("BH", msg_code, 0)            elif msg_code == 2:                path = raw_input("Enter path of wanted directory to get content of: ")                to_send = pack("BH"+'c' * len(path), msg_code, len(path), path)            elif msg_code == 3:                command = raw_input("Enter the wanted terminal command, including arguments: ")                to_send = pack("BH"+'c' * len(command), msg_code, len(command), command)            else:                print "Invalid message code, try again\n"            if 1 <= msg_code <= 5:                my_soc.send(to_send)        else:            break    data = my_soc.recv(1024)    unpacked_data = unpack("BH"+'c' * (calcsize(data) - 3), data)    print "The server's response to your type-"+ str(msg_code) +" request:"    print unpacked_data[2]    my_soc.close()def main():    client()if __name__ == "__main__":    main()