Expecting to find object/array on stack

I got it down to a few lines of code:

class Test {
def mdd = null
def add_table(table_name, spec, force = false, db = mdd) {
//def dbb = db
spec.each

{c, s -> db.insert_row("DB_MODEL", s) }

}
}
class DB {
def insert_row(str, data){}
}
[db_norms:[table:["type":"T_String50", "nullable": false]]].each{t, d->
new Test().add_table(t, d, false, new DB())
}

You can cut and paste it into the groovyConsole and it fails.

You can make it work either if you uncomment
def dbb = db
and change to
dbb.insert_row("DB_MODEL", s)

OR you change
def add_table(table_name, spec, force = false, db = mdd) {
to
def add_table(table_name, spec, force = false, db = null) {