Error using Matlab function in simulink

Hi all, I need to Model the CDF of a Gaussian in simulink, i defined this matlab function block:

function y = hplus(u)
 y = (1/2)*(1+erf((u(1)-MiuPlus)/(SigmaPlus*sqrt(2))));

But when i run it i obtain serveral errors:

    Simulink does not have enough information to determine output sizes for this block. If you think the
    errors below are inaccurate, try specifying types for the block inputs and/or sizes for the block outputs.

   Undefined function or variable 'MiuPlus'.
   Function 'Heating/h+' (#35.45.52), line 2, column 24:
   "MiuPlus"

   Errors occurred during parsing of MATLAB function 'Heating/h+'
   Component:MATLAB Function | Category:Coder error
   Simulink cannot determine sizes and/or types of the outputs for block 'Heating/h+' due to errors in the 
   block body, or limitations of the underlying analysis. The errors might be inaccurate. Fix the indicated 
   errors, or explicitly specify sizes and/or types for all block outputs.

   Component:MATLAB Function | Category:Coder error
   Simulink cannot determine sizes and/or types of the outputs for block 'Heating/h+' due to errors in the
   block body, or limitations of the underlying analysis. The errors might be inaccurate. Fix the indicated 
   errors, or explicitly specify sizes and/or types for all block outputs.

   Component:Simulink | Category:Model error
   Error occurred in 'SMA/Heating/h+'.

I don't understand why it doesn't find "MiuPlus" that is defined in the workspace, and have no idea of how to deal with the others errors.