[R-meta] Coding for meta-analysis.

When coding studies to be included in a meta-analysis (meta-regression) not
all of them will have the same covariates. My question deals with the
following situation.

One study has age and sex
One study has age, sex, and exposure to an infectious disease.

Would it be incorrect to code the first one as "0" because it doesn't have
the exposure variable, or should it be left as a missing variable?

Apologies for asking what is probably a "n00b" question. It has not been
addressed in Borenstein or Petitte. (I have more reading ahead of me.)

Thank you for your time and input.

Regards,

N. S. Miceli