IPN error: Missing POST variables. Identification is not possible.

I have followed the instructions on the video tutorial to set up the "PayPal Payments Standard Gateway" in the membership plugin.

It told me to setup an IPN listening URL with PayPal, and provided instructions, and gave me my own IPN listening URL.

But after following the instructions "Setting Up IPN Notifications on PayPal"

When I test the URL it gave me, I get this message:

"Error: Missing POST variables. Identification is not possible."

The payment is live mode, not sand box.

What have I missed?