Canceling statement results in broken connection (JDBC)

Using Statement.cancel() to cancel a running query, queries are
sometimes successfully canceled, but more often result in a broken
connection, after which all interactions result in an error such as
this:

Unknown Response Type c
     at org.postgresql.core.QueryExecutor.execute(QueryExecutor.java:123)
     at
org.postgresql.jdbc1.AbstractJdbc1Connection.ExecSQL(AbstractJdbc1Connec
tion.java:505)
     at
org.postgresql.jdbc1.AbstractJdbc1Statement.execute(AbstractJdbc1Stateme
nt.java:320)
     at
org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Stateme
nt.java:48)
     at
org.postgresql.jdbc1.AbstractJdbc1Statement.executeQuery(AbstractJdbc1St
atement.java:153)

The response type code is random, implying that the client is receiving
unexpected data from the back end.

PostgreSQL 7.3.4, Sun Java2 SDK 1.4.2, Windows (Cygwin).

Alexander.