Two comments
1 - the windowmode arguement for reports became available starting with the 2002 (xp) version - is that what you're working with?
2 - the windowmode arguement is the fifth arguement (should it be available in the version you're working wiht, see #1), you've put it as the fourth
Does any of this answer your question?
If you're working with a previous version, you might perhaps try something along the following lines
dim r as report
DoCmd.OpenReport rptName, acViewDesign
set r = reports(rptName)
r.visible = false