So I recently got a trade message for a mutagen. It was intended that the other party would gift this to me, but I guess they hit the wrong button. The seller left the Price field blank though and was able to still send the trade. I got a message with this text: You are offered to buy 1 Mutagen for .

When I tried to accept the trade however, I got this error message: 

• No bid specified!
• You don't have enough coins to make this purchase!

I also tried to create a trade with an empty Price field, and yes, it works. So I'm guessing we encountered a bug, that the Price field should not be allowed to be empty.