Can mxSE() calculate the standard error for a formula containing definition variables?

Hi everyone,

Hope you are enjoying the winter break!

I am trying to estimate a derived parameter using mxEval() and mxSE(). The function mxEval() works well, but mxSE() reports a warning message as below and produces NA for the standard error.

"1: In mxSE(paste0("instant_rate_est", j), model = model, ... :
Some diagonal elements of the repeated-sampling covariance matrix of the point estimates are less than zero or NA.
I know, right? Set details=TRUE and check the 'Cov' element of this object. "

I installed the last version of OpenMx and here are the lines I am using:
Loads1 <- mxMatrix("Full", 1, 1, free = F, labels = paste0(data.T1), name = "t1")
Loads2 <- mxMatrix("Full", 1, 1, free = F, labels = paste0(data.T2), name = "t2")
midT1 <- mxAlgebra(t2 - t1, name = mid_t1)
slp_est1 <- mxAlgebraFromString(paste0("mueta2 * mid_t1"), name = paste0("instant_rate_est", 1))

In the formula, "mueta2" is a free parameter, so my guess is it was caused by the definition variables "data.T1" and "data.T2", but I am not sure. Any advice on how to avoid such warning messages and have SE values?

Thank you so much for your attention.