I fit a basic latent growth curve model (using covariance data and means) and everything seems to work fine, but the RMSEA is returning NA in the summary function. Looking at the behavior of the summary function, it appears that this is because the squaredRMSEA is less than 0 (model chi-sq=3 with df=4). I am just now in the process of learning SEM, so forgive the basic question. But I thought that RMSEA should be given as 0 if this happens, not NA. I'm sure that I'm just missing something, but what is the reasoning behind returning NA?
Thanks in advance!