VCF header issue

[jodyphelan]

I ran into an issue when using bcftools to filter VCF files output from pilon.

$ bcftools filter -e 'FMT/DP<10' example.vcf
Error: cannot use arithmetic operators to compare strings and numbers

I think the issue is that DP is defined as a String rather than an Integer in the header

##FORMAT=<ID=DP,Number=1,Type=String,Description="Approximate read depth; some reads may have been filtered">

Is there a reason for this?