Using other files based on dynamic outputs with mixed non-dynamic wildcards in snakemake

Question

I am trying to use a Snakefile that does something like this:

rule split_files:
    input:
        '{pop}.bam'
    output:
        dynamic('{pop}_split/{chrom}.sam')
    shell:
        "something"

rule work:
    input:
        sam='{pop}_split/{chrom}.sam',
        snps='snps/{chrom}_snps'
    output:
        '{pop}_split/{chrom}_parsed.sam'
    shell:
        "something"

rule combine:
    input:
        dynamic('{pop}_split/{chrom}_parsed.sam')
    output:
        '{pop}_merged.sam'
    shell:
        "something"

This results in the error:

Missing input files for rule work:
snps/__snakemake_dynamic___snps

Adding dynamic to both of the inputs for the work rule results in the same error.

I need to do this because some populations have chrY and others do not, so I can't just expand with a chromosome list (actually I can with another work around that I am currently using, but it is cumbersome).