'Internal error' when calling outer procedure after deleting unused result variable from inner procedure

[ppazera]

Firebird 5.0.3 classic on 64 bit windows
Firebird 5.0.2 produces ‘conversion error from string "20-NOV-1858 0:00:00.0003"’ error
Firebird 2.5.9 works fine
Error happens for this and other connections

Here is test case to reproduce:

set term ^;
create or alter procedure sp1
returns (field1 integer, field2 integer)
as begin
  field1 = 1;
  field2 = 2;  
  suspend;
end 
^
create or alter procedure sp2
returns (field1 integer) 
as begin
  field1 = 1;
  if (exists(select 1 from sp1 where field1 = 2)) then
    suspend;
end 
^
set term ;^
commit;

select * from sp2;
commit;

set term ^;
create or alter procedure sp1
returns (field1 integer) 
as begin
  field1 = 1;
  suspend;
end 
^
set term ;^

/*following errors out in this and other connections*/
select * from sp2; 
/*5.0.2.1613:
Statement failed, SQLSTATE = 22018
conversion error from string "20-NOV-1858 0:00:00.0003"
-At procedure 'SP2' line: 5, col: 3*/
/*5.0.3.1683:
Statement failed, SQLSTATE = XX000
internal error
-At procedure 'SP2' line: 5, col: 3*/

drop procedure sp2;
drop procedure sp1;
commit;