Spontaneous logical index creation addKeyFigure error

Hello,

I have the following two queries:

SELECT AVG(LENGTH(bio)) FROM members WHERE bio IS NOT NULL;
SELECT MEDIAN(LENGTH(bio)) FROM members WHERE bio IS NOT NULL;

The members table looks like this:

CREATE COLUMN TABLE members (    id INTEGER,    bio NCLOB,    ...    PRIMARY KEY (id)
);

Now, when I execute the two queries from above via the JDBC-Driver, the following error occurs:

com.sap.db.jdbc.exceptions.JDBCDriverException:
SAP DBTech JDBC: [2048]: column store error:
logical index creation addKeyFigure error:  
[2030] Adding key figure failed: Lob type attribute 'SCHEMA.MEMBERS.BIO$aggr$' not supported

This happens only for the AVG query. MEDIAN works fine. Furthermore, on a different table with the exact same definition of an attribute "bio" the first query works like a charm. How can this be?