Not able to delete series from measurement with tag is WHERE clause

I have a measurement in my database with 2 tag values for a tag key.
To verify that this is a tag:

show tag values from prometheus_tsdb_head_series with key=business_unit
name: prometheus_tsdb_head_series
key value

business_unit ABC
business_unit XYZ

But when I try to delete it I see:
DROP SERIES FROM “prometheus_tsdb_head_series” where “business_unit” = ‘ABC’
ERR: shard 33: fields not supported in WHERE clause during deletion

The WHERE clause condition is a tag but why is InfluxDB complaining its a field?