[From string (line 5) ]                                                                                   
                                                                                                          
@startuml                                                                                                 
!global $table = "|= Field1 |= Field 2 |"                                                                 
!function $row($value1, $value2)                                                                          
%set_variable_value("$table", %get_variable_value("$table") + " \n | " + $value1 + " | " + $value2 + " |")
!endfunction                                                                                              
^^^^^                                                                                                     
 This function does not have any !return directive. Declare it as a procedure instead ?