Cannot convert from cvx to double

My Statement:
variable sigma(1,1) %%% sigma:对应 σ
variable eta(1,1)
constraint(1) = power(2,eta)-sigma;

My Constraints:
subject to

During the iteration process, the first iteration runs normally without errors. But on the second iteration, the following error occurred:
The following error occurred when converting from cvx to double:
Cannot convert from cvx to double.

What’s the matter, please? Hope to get your answer, thank you

Hi,Mark, thank you very much.After listening to your suggestion, I defined an expression, and then the code can run normally. Thank you very much for your help!