How to get a multi match query to fail gracefully when a field doesn't exist

I'm using the following query:

dis_max: {
   queries: [
     {"multi_match": { "fields": ["b.*"], "text": "foo", "fuzziness": 2},
    {"multi_match": { "fields": ["a"], "text": "foo", "fuzziness": 0}

Where a is a required field, but b.* is not.

It works fine most of the time, but when I start my application from scratch, I can get into a state where nothing under b gets indexed. And since I'm using dynamic mappings for b, this means that the query fails with:

! org.elasticsearch.index.query.QueryParsingException: No fields specified for multi_match query

I'd like to tell the second multi_match query to simply not match when the wildcard doesn't expand to any known fields.

Is this possible?