Google Genomics - problem with regions/zones settings

While trying to run the GATK example at
https://cloud.google.com/genomics/v1alpha2/gatk
I get the following error
Unsupported key/value pair in WorkflowOptions: defaultRuntimeOptions -> {"zones":"us-central1-b us-central1-c"}
see also the stderr file attached.
So it looks like there is an problem with the region/zone settings. As I am running this from Belgium, I tried switching to the european zones in wdl/scripts/broad_pipelines/PublicPairedSingleSampleWf_160927.options.json but that did not seem to work