Complex Samples -- subpopulations in Chi-Square

I have a survey design that I've implemented using the Complex Samples module. Running regressions, logit, etc, work fine with subpopulations specified. However, when I attempt to run a chi-square test using crosstabs I get the following warning:

"The Test of Independence is not available because the subpopulation variable: [varname] is not specified as a stratification variable of the first stage."

Can anyone shed any light on why this comes up here, but not in the context of a linear model? I've been comparing this all with stata code/output, and find no such limitation in that package- is this specific to SPSS' Complex Samples module?

Many thanks.