[NTG-context] blackrule

Dear Sirs, I want to prepare a forma for handwriting filling. With this propose e use \blackrule command for trace some lines inside paragraphs. For example, I, \blackrule[width=5cm, height=0.05cm] , promise maintain a good... In the pdf document appears: I, !max___________________ , promise maintain a good... Are there a good reason for that? Thanks Jorge