why is there a value for effect size (g) but return NaN for variance?

[Elie-Yu]

There is no missing value in my dataset for the pretest, posttest mean, sd, and sample size. However, when I used the function esc_mean_gain(), some rows return with g and variance of g, but some other rows return only value for g but NaN for variance of g.

For example, the below is one example: d<-esc_mean_gain(pre1mean = 3.52, pre1sd = 0.56, post1mean = 3.52, post1sd = 0.65, grp1n = 34, pre2mean = 3.5, pre2sd = 0.61, post2mean = 3.81, post2sd = 0.67, grp2n = 40, es.type = "g")

#check results d$es = 0.49 d$var = NaN

Please help!