"Integers can only be combined with integers of the same class, or scalar doubles" Error

i am getting this error for the below code. Can anyone help with this. the elements of c1 are of data type int32. I cannot change that size. Is there any other way to get through this.

Integers can only be combined with integers of the same class, or scalar doubles.

Error in trial>decryption_Callback (line 454)

m1(l)=mod(c2,n);

 c1=reshape(c,1,[]);

    s=numel(c1);

    m1=c1;

    for l=1:s

        c2=c1(l)^msk;

        m1(l)=mod(c2,n);

    end

    m=char(m1);