This kind of error message generally comes up (in any MATLAB product) when you are trying to call a function but have shadowed the function name with a variable name. For example, if you do this:
rosgenmsg = 1;
myPath = 'C:\ros';
MATLAB is trying to index into the variable "rosgenmsg" using the character values in "myPath" as index values. Since 'C' is valued at a larger index than the length of "rosgenmsg", you see this error.
Run this command to see if you have "rosgenmsg" shadowed by a variable name: