Drawing Complex Sample with unequal strata sizes returns warning in SPSS Statistics

Question

I am creating a sample from a state survey file, sampling without replacement from several zip codes that represent health region service areas. ZIPCODE is my strata variable. The size of the sample in each zip code varies with the population size of the zip code. I have designed the sampling plan file, based on the sampling strategy, and I now want to create a separate dataset that includes only the sampled cases. In the Sampling Wizard (Analyze->Complex Samples->Select a Sample), I choose "Draw the sample" and specify the sampling plan file. However, the new sample data set is not created and I receive the following warning in my output viewer: "The plan file does not provide sample size specifications for the following stratum at stage 1: Zipcode = . Execution of this command stops. There are no valid cases. Statistics cannot be computed." The zipcode value that was cited in the warning was not a zip code in my list of desired strata so I did not specify a sample size. Why would this omission derail the sample selection?