The line of code you shared looks okay.  If you are still having problems, share a reproducible example that can be de-bugged.

Two things here:

1) The variable name should remain the same throughout the code. In the model, you have written:

fit

Dear Shreyas,

Thanks for your help.

Regarding 1), in my code the variable is the same throughout the code. Effectively, it was a mistake that I did when I wrote the message in research gate. 

Considering 2),  I have already tried to convert to a factor the variable "factor 1" at the beginning of the code  as follows:

data.frame$factor 1