Function does not work for an nlmer model

[jknowles]

Via email:

Error in envRefInferField(x, what, getClass(class(x)), selfEnv) ‘family’ is not a valid field or 
method name for reference class “nlsResp”

> traceback()
6: stop(gettextf("%s is not a valid field or method name for reference class %s",
       sQuote(field), dQuote(thisClass@className)), domain = NA)
5: envRefInferField(x, what, getClass(class(x)), selfEnv)
4: merMod@resp$family
3: merMod@resp$family
2: predictInterval(merMod = nlmer.model, newdata = newdata_mix,
       level = 0.95, n.sims = 1000, stat = "median", type = "linear.prediction",
       include.resid.var = F, which = "fixed") at #14

Is it possible to make the package work for an nlmer model or class?