Here's the error from the V7 manual:
-873 DATA ENCODED WITH DIFFERENT ENCODING SCHEMES CANNOT BE REFERENCED
# IN THE SAME SQL STATEMENT
| Explanation: You cannot refer to a column defined in a table in one
| encoding scheme in the same SQL statement as a column defined in a table
| of another encoding scheme.
| This situation can occur when a table created in either the ASCII, EBCDIC,
| or UNICODE encoding schemes is referenced in a statement with a table that
| is not in the same encoding scheme.
This situation can also occur when a table is created; a DECP CCSID value
is changed; another table is created, and then the two tables are
referenced in a single SQL statement.
System Action: The statement cannot be executed.
Programmer Response: Correct the SQL statement.