"not in a transaction", but only sometimes

[Laura Morales]

Just out of curiosity, has someone else here experienced a query that returns 
"not in a transaction" only sometimes?
I have a simple dataset like this


    :dataset a ja:RDFDataset ;
        ja:defaultGraph :union

    :union a ja:UnionModel ;
        ja:subModel :graph1 #TDB
        ja:subModel :graph2 #TDB
        ...


I have a DESCRIBE query that works. However when I submit it from my program 
after other SELECT/ASK queries in sequence, it stops working. I see this error 
"Not in a transaction", and I have to restart Fuseki. All my databases in the 
union are TDB1, not TDB2.