coplots with missing values

Hello - I'm trying to construct a coplot to check the residuals conditioned
on a factor using the following formula, however I keep getting an error
message.  I've used the same formula for another data set and it worked fine
- the only difference is that now I have missing values for each level (2
levels) of the factor. Is this the problem and how can I fix it.  Thank you.

E2 <- resid(baea.ancova6, type = "normalized")
coplot(E2~year|population)

Error in bad.lengths() : incompatible variable lengths

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