[R] na's and continuous variables

[Tonker]

Hi 
I am performing a lmer on count data with several explanatory variables both
continuous and categorical. All go in fine apart from two of the continuous
variables that have na's in them. They give errors like:

Leading minor of order 5 in downdated X'X is not positive definite

I used the as.factor function to define my categorical factors as
categorical but when i ask is.factor about my two continuous variables that
i didnt use the as.factor for it replies yes.
I have so far come to conclusion that because the data contains na's R is
automatically reading it as categorical not continuous and i was wondering
if there is a way to overcome this?

Thanks
Tonker

ps i do use the na.action=na.omit in my model so if possible i would like to
not have to delete all the records with na in in these two explanatory
factors.
 
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