R help - Apr 2011 - mlogit package, "Error in X[omitlines, ] <- NA : subscript out of bounds"

I am using the mlogit packages and get a data problem, for which I
can't find any clue from R archive.

code below shows my related code all the way to the error

#---------------------------------------------------------------------------
mydata <- data.frame(dependent,x,y,z)

mydata$dependent<-as.factor(mydata$dependent)

mldata<-mlogit.data(mydata, varying=NULL, choice="dependent",
shape="wide")

summary(mlogit.1<- mlogit(dependent~1|x+y+z, data = mldata,
reflevel="0"))

"Error in X[omitlines, ] <- NA : subscript out of bounds" ,
#---------------------------------------------------------------------------

Could anybody kindly tip how  can I possibly solve this problem?

Thank you

yong