Cannot load message class for [].

edit

I want to publish the message from remote PC to Raspberry Pi.

First of all, I published the message and tested the Raspberry to make sure that the publisher and Pi worked and the message has been correctly established. Then I connected my PC with Raspberry Pi and prepared all such things as source and ROS_MASTER_URI and so on. After that I typed rosmsg listand the message was just there. I think it was OK But when I typed rostopic echo /mytopic or rostopic pub -r 10 ... just got the ERROR Cannot load message class for...

Could anyone kindly tell me how to fix this error? Thanks a lot