dredge function error - R package MuMln

See ?dredge:

# Example from Burnham and Anderson (2002), page 100:
data(Cement)
options(na.action = "na.fail")   #  prevent fitting models to different datasets

fm1 <- lm(y ~ ., data = Cement)
dd <- dredge(fm1)

If you skip the second line, your described error pops up, as the models are fitted to different datasets (due to removal of NAs).

The issue with using options(na.action = "na.fail") is that it changes the global settings of R. If you have a large script, changing the global settings will potentially impact on other sections of your code where you implicitly rely on R's default settings. There are two ways to avoid this:

1. After using dredge change the settings back via options(na.action = "na.omit").

OR the better way...

2. Utilise the regression function's ability to set the na.action argument. In your case:

glm1 <- glm(presabs ~ dca1+dca2+se1+se2,
            family=binomial(logit),
            na.action = "na.fail")