I-539 for spouse with my valid I-94

[user111]

I am on E2 visa with my family as my dependents. I have a valid i-94 (which I obtained few months ago in one of my business trips to canada).

I applied for i539 extension for my spouse.

USCIS sent me a letter stating that they need a copy of my I-797.

I do not have I-797, nor do I need it because my I-94 is valid for 2 years.

Any thoughts what I should do?

thank you
Simon