Inconsistent stack height with "if" in closure

The script

token = null
closure = {
if (token == null) token = "hi"
return token
}
println closure.call()

fails with an Inconsistent stack height 1 != 0 error. It fails no matter what the format of the "if" statement. It also fails if the closure is a function instead, like this:

def foo(str) {
token = null
str.each

return token
}
println foo()