Insert into linkmap via subselect

Hi,
I tried to insert a new row, where the linkmap property ist generated by a subselect:

CREATE CLASS Day;
CREATE CLASS Hour;
CREATE CLASS Log;
CREATE PROPERTY Day.hour LINKMAP Hour;
CREATE PROPERTY Day.ID integer;
CREATE PROPERTY Hour.log LINKSET Log;
CREATE PROPERTY Hour.ID integer;

Without a subselect it works fine:
INSERT INTO Day(ID,hour) VALUES (1,{“15”:#19:0});
With subselect
INSERT INTO Day FROM (SELECT 1 AS ID,"{“15”:"+@RID.convert(“string”)+"}" AS hour FROM Hour WHERE ID=1);
Errormessage is:
Error: com.orientechnologies.orient.core.exception.OValidationException: The field ‘Day.hour’ has been declared as LINKMAP but an incompatible type is used. Value: {“15”:#19:0}

Thanks

Robert