Bug in TAM::IRT.truescore for single item

[cc20002002]

This code will produce the bug

data(data.Students, package="CDM")
dat <- data.Students[, paste0("mj",1:4) ]

# fit partial credit model
mod1 <- TAM::tam.mml( dat,control=list(maxiter=20) )
summary(mod1)

# true score conversion with user-defined theta grid
tmod1b <- TAM::IRT.truescore( mod1,1, theta=seq(-1) )

A workaround can be

tmod1b <- TAM::IRT.truescore( mod1,c(1,1), theta=seq(-1) )$truescore/2