Issue with s(date)

[louislegum]

Hi there,
I have been using the stm package, when I use s(date) as a covariate for prevelence I get this error message (see below) but I dont get the same message when not applying the s() function and I was wondering how important this difference is and what may be causing the issue?

Error message:
Error in makeTopMatrix(prevalence, data) : Error creating model matrix.
This could be caused by many things including
explicit calls to a namespace within the formula.
Try a simpler formula.

STM code:
first_stm <-
stm(documents = q_dfm1$documents,
vocab = q_dfm1$vocab, K = 15, prevalence =~ Newspaper_regionality + s(Date), max.em.its = 150,
data = q_dfm1$data, init.type = "Spectral", verbose = FALSE)

Thank you for your time!