NEW YORK (CNNMoney) -- If you want to attend Google's annual developer conference, Google I/O, you'd better be prepared to use the company's technology to secure a spot.
When it opened registration on Tuesday, Google required potential attendees to use a Google+ account and pay the $900 general admission fee using Google Wallet, the company's new -- and not exactly widely adopted -- payment system.
It's a mandate that drew criticism from some in the tech realm. In a blog post titled "Why I'm letting my Google IO Invitation expire," entrepreneur Jeffrey Zeldman blasted the obstacles involved in signing up.
Google itself acknowledged the potential hurdles."In case your bank declines your purchase through Google Wallet, you may need to call the bank that issued your credit card and let them know that you want to make a large purchase," the company wrote in an email to potential attendees.
Google (GOOG, Fortune 500) spokeswoman Elizabeth Markman defended the Google Wallet requirement, saying it was intended to speed things up.
"Because you can configure information ahead of time, we used Wallet to expedite the registration process," she said, noting that it's not the first time Google required registrants to use its own tech services. It required Google Checkout, the predecessor to Google Wallet, for Google I/O registration in 2011.
Google Wallet is intended to be a phone-based payment system, but right now it's only available on a small number of Sprint phones. A Web version has a broader potential audience, but even there, the service not available yet in some countries. To register for Google I/O, people in those countries had to call or email Google for assistance.
"The number there was minimal," Markman said.
It may have been a hassle, but the Google Wallet requirement clearly wasn't a dealbreaker. The conference sold out in 20 minutes, Google executive Vic Gundotra announced on Google+. The gathering will take place in late June in San Francisco. 
