Reader query on avoiding all-male colloquia

A reader writes:

I’m a Ph.D student in a philosophy department and was recently tasked with organizing a graduate student colloquium series for the upcoming semester. I sent out an email calling for volunteers, got quick responses, and typed up the list (the speakers and dates are now finalized). Then it struck me all too late that the list was completely male.

It should be noted that none of the women in the department volunteered to present–but I think it would be premature for me to write this off as their own fault for not volunteering. It’s too late for me to change the list now, but I am wondering if you can suggest any ways to avoid this in the future. Merely telling the women in the program that they should volunteer because we need to diversify the list seems heavy-handed to me.

Any thoughts on what the organiser should do? My thought would be to make individual suggestions to women of the form “Hey, you gave a great paper on X to our work in progress seminar. Maybe you could do that at the colloquium?”.