Problem calling setweight function from JDBC

Hello,

I have a call to setweight function in a PreparedStatement with the following sql:

update my_table set a_text_data=setweight(to_tsvector(? :: regconfig, ?), ? :: char)

Later I set the parameters:
stmt.setString(1, language);
stmt.setString(2, textToIndex);
stmt.setString(3, weight);

But the call fails with:

"ERROR: function setweight(tsvector, character) does not exist"

I had very similar error with ts_vector where the cast to regconfig helped, but not in this case. Inspecting the
setweightfunction in system catalogs shows that parameters are tsvector and char. What am I doing wrong? 

Thanks in advance,
Viliam