Is the forward slash "/" a reserved character in solr field names?

I'm having trouble writing a solr sort query which will parse for fields containing a forward slash "/"

When making an http query to my solr server:

q=*&sort=normal+desc

Will work but

q=*&sort=with/slash+desc
q=*&sort=with%2Fslash+desc

Both fail say "can not use FieldCache on multivalued field: with"

Each solr document contains two int fields "normal", and "with/slash". With my solr schema indexing the fields as so

...
<field name="normal" type="int"   indexed="true" stored="true" required="false" />
<field name="with/slash" type="int"   indexed="true" stored="true" required="false" />
...

Is there any special way I need to encode forward slashes in solr? Or are there any other delimiter characters I can use? I'm already using '-' and "." for other purposes.

lucene solr
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