remove_constraints!(opt) doesn't work; chkn() not defined

[gwhammett]

I'm finding NLopt.jl very useful, but when I try using the remove_constraints!(opt) function, it doesn't work. Running the first example (using the standard interface, not the JuMP interface) in the README.md file (at https://github.com/JuliaOpt/NLopt.jl) works just fine, but if I follow it with the command:
remove_constraints!(opt)
it gives the error message

UndefVarError: chkn not defined

Stacktrace:
 [1] remove_constraints!(::Opt) at /Users/myname/.julia/packages/NLopt/eqN9a/src/NLopt.jl:457
 [2] top-level scope at In[13]:1

The offending line in function remove_constraints!(o::Opt) is:

chkn(ccall((:nlopt_remove_inequality_constraints,libnlopt),
               Result, (_Opt,), o))

It appears that the chkn() function is not defined. I'm not an expert at Julia and am not sure how to fix this. Is there another module where it is defined that I should also be using?