Error message after grm (package "ltm") for item response theory

library(ltm)

ITFdata <- alea1
modelGRM <- grm(ITFdata[c(1:60)], constrained=TRUE, IRT.param=FALSE)
modelGRM

modelGRM <- grm(ITFdata[c(1:60)], constrained=TRUE, IRT.param=FALSE)
Erreur dans log.pr[xj, ] : indice hors limites

Any idea how to fix this problem, and what is the problem ?

Thx