bad formfield error message : "THIS COMMAND IS UNAVAILABLE...."

[MiK@GenApi]

THIS COMMAND IS UNAVAILABLE BECAUSE THE FORM FIELD WAS NOT INSERTED WITH THE
FORMS TOOLBAR OR BY USING THE INSERT FORM FIELD DIALOG BOX

I sometimes get this boring error message when I try to open some
formfields' properties dialog box in a word document. But those fields have
been yet created with the Word forms toolbar. I've got that problem randomly,
so I cannot really try to reproduce it and debug.

Would anybody know the cause, and how could I solve it?

Thank's for your help.