[R-sig-ME] error from clmm

[Malcolm Fairbrother]

Dear list,

I am making use of the "clmm" function from the "ordinal" package, for fitting mixed models with ordinal outcomes.

When fitting quite a few models, I get the error:

Error in if (maxGrad < rho$ctrl$gradTol) { : 
  missing value where TRUE/FALSE needed
In addition: Warning message:
In sqrt(phi2) : NaNs produced

Can anyone help me to understand why this may be happening, and/or whether there's anything obvious I should be doing different? With some seemingly small/arbitrary specification changes--or when taking (some) random subsets of my dataset--I don't get this message. But I'd like to find a way of reducing the frequency with which I do get it.

I don't think there's anything unusual about my call:

mod1 <- clmm(as.ordered(y) ~ x + (1 | fac1) + (1 | fac2), data=dat, link="logit", threshold="symmetric")

I'm running:
R version 2.15.0 (2012-03-30)
Platform: x86_64-apple-darwin9.8.0/x86_64 (64-bit)

Many thanks to anyone who can assist,

Malcolm