Could someone explain this error on fprintf() in Octave?

Question

I am trying to write in a text file the result of an integral, but when I run the program I get the next error.

error: fprintf: wrong type argument 'class'

Here´s my code.

syms x
f=x;

archivo=fopen('ejemplo.txt','w')

variable=int(f,x,1,2)

fprintf(archivo,'%f\n',variable)


fclose(archivo)

Could someone explain whats wrong?