Length of escape string in LIKE ... ESCAPE not properly checked with territory based collation

When an ESCAPE clause contains multiple characters, but only a single collation element, they are accepted as long as it's not a literal:

ij> connect 'jdbc:derby:memory:nodb;create=true;territory=no;collation=TERRITORY_BASED';
ij> create table t(x varchar(20), y varchar(20), z varchar(20));
0 rows inserted/updated/deleted
ij> insert into t values ('abc', 'def', 'aa');
1 row inserted/updated/deleted
ij> select * from t where x like y escape z;
X |Y |Z
--------------------------------------------------------------

0 rows selected

If the same query is performed with a literal in the ESCAPE clause, the problem is detected and correctly reported:

ij> select * from t where x like y escape 'aa';
ERROR 22019: Invalid escape sequence, 'aa'. The escape string must be exactly one character. It cannot be a null or more than one character.