"zero__" in conditional_effects() output

I’d like to get the conditional effects of a model assuming that a number of variables are unknown or missing (i.e., NA) I tried to condition them as NA by creating a data.frame with the variable name as follows:

conditions1 ← data.frame(some_vars = NA, some_vars2 = NA, some_vars3 = NA)

When I add this condition to the conditional_effects function, the output given was it was conditioned on “zero__” as opposed to NA. Is this the correct output or was brms assuming a certain value when it was set as missing for conditional_effect function?