Error when running mle2 function (bbmle)

Question

I am receiving the following error when running the mle2() function from the bbmle package in R:

some parameters are on the boundary: variance-covariance calculations based on Hessian may be unreliable

I am trying to understand if this is due to a problem with my data or an issue with calling the function properly. Unfortunately, I cannot post my real data, so I am using a similar working example of the same sample size.

The custom dAction function I am using is a softmax function. There have to be upper and lower bounds on the optimization so I am using the L-BFGS-B method.

library(bbmle)
set.seed(3939)

### Reproducible data
dat1 <- rnorm(30, mean = 3, sd = 1)
dat2 <- rnorm(30, mean = 3, sd = 1)
dat1[c(1:3, 5:14, 19)] <- 0
dat2[c(4, 15:18, 20:22, 24:30)] <- 0

### Data variables
x <- sample(1:12, 30, replace = TRUE)
pe <- dat1
ne <- dat2

### Likelihood
dAction <- function(x, a, b, t, pe, ne, log = FALSE) {
  u <- exp(((x - (a * ne) - (b * pe)) / t))
  prob <- u / (1 + u)

  if(log) return(prob) else return(-sum(log(prob)))
}

### Fit
fit <- mle2(dAction,
            start = list(a = 0.1, b = 0.1, t = 0.1),
            data = list(x = x, pe = pe, ne = ne),
            method = "L-BFGS-B",
            lower = c(a = 0.1, b = 0.1, t = 0.1),
            upper = c(a = 10, b = 1, t = 10))

Warning message:
In mle2(dAction, start = list(a = 0.1, b = 0.1, t = 0.1), data = list(x = x,  :
  some parameters are on the boundary: variance-covariance calculations based on Hessian may be unreliable

Here are the results for summary():

summary(fit)
Maximum likelihood estimation

Call:
mle2(minuslogl = dAction, start = list(a = 0.1, b = 0.1, t = 0.1), 
    method = "L-BFGS-B", data = list(x = x, pe = pe, ne = ne), 
    lower = c(a = 0.1, b = 0.1, t = 0.1), upper = c(a = 10, b = 1, 
        t = 10))

Coefficients:
  Estimate Std. Error z value Pr(z)
a      0.1         NA      NA    NA
b      0.1         NA      NA    NA
t      0.1         NA      NA    NA

-2 log L: 0.002048047 

Warning message:
In sqrt(diag(object@vcov)) : NaNs produced

And the results for the confidence intervals

confint(fit)
Profiling...

  2.5 %    97.5 %
a    NA 1.0465358
b    NA 0.5258828
t    NA 1.1013322

Warning messages:
1: In sqrt(diag(object@vcov)) : NaNs produced
2: In .local(fitted, ...) :
  Non-positive-definite Hessian, attempting initial std err estimate from diagonals