Question on semiparametric bootstrap in lme4

Hi,

I have a question on the semiparametric bootstrap result for lme4.
Specifically, the bootstrap standard deviation for the fixed effect is
essentially zero. Here is an example:

require(lme4)
fm01 <- lmer(Yield ~ 1|Batch, Dyestuff)
set.seed(1)
require(boot)
boo01_sp <- bootMer(fm01, fixef, nsim = 100, use.u = TRUE,

+                  type = "semiparametric")

boo01_sp

Call:
bootMer(x = fm01, FUN = fixef, nsim = 100, use.u = TRUE, type =
"semiparametric")


Bootstrap Statistics :
    original       bias     std. error
t1*   1527.5 9.094947e-13 1.392467e-12

Then I took a look on the source code for the function `bootMer`, and
found the relevant code:

if (type == "parametric") {
        ss <- simulate(x, nsim = nsim, use.u = use.u, na.action =
na.exclude)
    }
    else {
        if (use.u) {
            if (isGLMM(x))
                warning("semiparametric bootstrapping is questionable
for GLMMs")
            ss <- replicate(nsim, fitted(x) + sample(residuals(x,
                "response")), simplify = FALSE)
        }
        else {
            stop("semiparametric bootstrapping with use.u=FALSE not yet
implemented")
        }
    }

I notice that the semiparametric method is using sampling without
replacement (i.e., `sample(residuals(x, "response"))`, which is
different from what I learned about bootstrap. Should the `replace =
TRUE` argument be added?

Mark