Required to tell opponent my door card at Mirage

I was playing 5-10 stud at the Mirage today and the dealer made a ruling which almost caused me to get into my first-ever arguement with a dealer.


I limped in with rolled-up 5s (Later, I'll be posting another topic in the "Other Poker Games" section about whether you should limp or raise with small trips). A player behind me raised on 3rd street with a King showing and I called.


On 4th street, I got a 7. On 5th street, I got another 7 to make 5s full. I was heads up against the 3rd street raiser and bet since my board was high.


At this point, my opponent asked me what my door card was. I asked the dealer if I was required to answer his question under the assumption that the dealer certainly would not require me to do so. To my surprise, the dealer told me I had to tell my opponent what my door card was. I asked again and emphasized the word REQUIRED and the dealer told me I must tell my opponent my door card.


I was stunned and would have called a floor person over to make a ruling if it weren't for the fact that my opponent probably feared trip 7s. So I just told him the truth- the 5 was my door card. I assume he put me on two pair rather than trips and cetainly not a full house.


The hand continued and my 5s full won a decent pot.


But I still can't believe the ruling made by the dealer. Is this common practice? How could I be required to help my opponent by telling him the order my cards were received in?


And at the Mirage?