Dbm-update fails if datasource is defined in external config

We have our local development datasources defined in an external config.groovy. That file is then included in the configuration with the line grails.config.locations = [ "file:$

/.<appname>/Config.groovy" ] in the primary grails Config.groovy.

When I run dbm-update with the environment that depends on that, the script fails after printing out:
[echo] Starting dbm-update for database [:] @ [:]
It actually doesn't even appear to be loading the primary Grails Config.groovy since we have some printlns in Config.groovy to print out the current environment etc. and those print-outs aren't happening.