[R-sig-ME] glmer error: contrasts can only be applied to factors of 2 or more levels

[Dan McCloy]

I'm getting the following error, despite the fact that all my fixed
effects were converted to factors and had their contrast attributes
set prior to calling glmer.

Error in `contrasts<-`(`*tmp*`, value = contr.funs[1 + isOF[nn]]) :
  contrasts can be applied only to factors with 2 or more levels
Calls: glmer ... model.matrix -> model.matrix.default -> contrasts<-

Here is the model specification:
glmer(press ~ truth*adj*idn*num + (1|subj), data=wl,
family=binomial(link="probit"))

Here are the predictors. As you can see, they are all already factors
and have contrasts set, so I can't figure out why glmer is trying to
set contrasts anyway, much less why it thinks any of these have fewer
than two levels:

R> head(wl$truth)
[1] neither neither neither neither neither neither
attr(,"contrasts")
        target foil
neither      0    0
target       1    0
foil         0    1
Levels: neither target foil
R> head(wl$adj)
[1] TRUE TRUE TRUE TRUE TRUE TRUE
attr(,"contrasts")
      TRUE
TRUE     1
FALSE   -1
Levels: TRUE FALSE
R> head(wl$idn)
[1] FALSE FALSE FALSE FALSE FALSE FALSE
attr(,"contrasts")
      TRUE
TRUE     1
FALSE   -1
Levels: TRUE FALSE
R> head(wl$num)
[1] six six six six six six
attr(,"contrasts")
      three
three     1
six      -1
Levels: three six

Here is the session info:
R version 3.1.1 (2014-07-10)
Platform: x86_64-pc-linux-gnu (64-bit)

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
 [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
LC_PAPER=en_US.UTF-8       LC_NAME=C
 [9] LC_ADDRESS=C               LC_TELEPHONE=C
LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] lme4_1.1-7   Rcpp_0.11.2  Matrix_1.1-4

loaded via a namespace (and not attached):
[1] grid_3.1.1      lattice_0.20-29 MASS_7.3-34     minqa_1.2.3
nlme_3.1-117    nloptr_1.0.4    splines_3.1.1     tools_3.1.1

Can anybody shed some light on what is going on here, and / or how to
work around it?  I've run similar models with very similar data on
slightly older versions of lme4 (but still >1.0) and never run into
this, so I'm wondering if a bug was introduced in the latest version.
-- dan

Daniel McCloy
http://dan.mccloy.info/
Postdoctoral Research Fellow
Institute for Learning and Brain Sciences
University of Washington