Error with function SimContinuous of Simpop package

Hi,
I have created a synthetic population based on a sample with Simpop package and now I'm trying to create a continuous variable with SimContinuous function as follows:

model=~th+cnae
msintcont<-simContinuous(msintsup1, additional="age", method="lm", regModel=model, zeros=FALSE)

where msintsup1 is a simpop object, and th and cnae are the predictor variables. Both are factors with 3 and 90 levels respectively, and both are in the sample and in the synthetic population.

When I run this code, I obtain this error:

Error en {:
task 1 failed - "contrasts can be applied only to factors with 2 or more levels"

How could I solve this error? The predictor variables both have more than 2 levels...

Than you in advance!