Kibana Dashboard syslog ECS

Hello,
I get the following message in my Kibana Dashboard.
"1 of 2 shards failed. The data you are seeing might be incomplete or wrong."
The message disappears after 30 seconds and only comes back when i refresh the Dashboard.

illegal_argument_exception
Reason
Text fields are not optimised for operations that require per-document field data like aggregations and sorting, so these operations are disabled by default. Please use a keyword field instead. Alternatively, set fielddata=true on [host.hostname] in order to load field data by uninverting the inverted index. Note that this can use significant memory.

How do I get rid of this message?