By the nature of how montone models work, you will not be able to impute the first variable in the list because there are no variables prior to it that can be used in its imputation model.   That being said, you might consider, if you have missing values in the first variable, using the FCS method instead. ... View more

Just want to add one more piece of information: I have tried the same model to generate multiple tables for different endpoints - it is odd that only one single table that had such problem: the order of the 'Base' in the model statement changed the standard error. All other tables had no difference.  ... View more