[R] likelyhood maximization problem with polr

[blackscorpio]

Dear community,

I am currently trying to fit an ordinal logistic regression model with the
polr function. I often get the same error message :

"attempt to find suitable starting values failed", for example with :

require(MASS) 
data(iris) 
polr(Species~Sepal.Length+Sepal.Width+Petal.Length+Petal.Width,iris) 

(I know the response variable Species should be nominal but I do as levels
were ordered for the example). 
I think this is a likelyhood maximization problem ; I tried to solve this by
setting the "start" option of polr to a null or a random vector by it
doesn't garantee to find "a good" solution at the end. 

Does anyone have a clue ?

Thanks a lot ! 
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