displaying a variable value in flashdata

I am trying to display the value of a id given to a class via a flash message.  Currently is is only displaying the string section of the message ie. "Your class code is: " so the issue is the concatenation of the string and the variable. However, when i have tried just displaying the variable without the string section nothing appears. Do I need to pre-process the variable before I set it in the flash data?

The code is:

$this->session->set_flashdata('class_id', ('Your class code is: '.$class_Id));