Confirmed with MuMIn 1.9.5 ... it looks like there is a bug in the beta.weights()
function.
The fourth line,
coefmat <- coefTable(model)[, 1L:2L]
should be
coefmat <- coefTable(model)[, 1L:2L, drop=FALSE]
You can fix this for yourself, temporarily, as follows:
fix(beta.weights)
## opens an editor; make the change in the editor and save/exit
assignInNamespace("beta.weights",beta.weights,"MuMIn")
However, it would definitely be a good idea to contact the package maintainer (see help(package="MuMIn")
and submit a bug report ...