Strange issue with query design function in access - sql

Question

I have the error message "Data type mismatch in criteria expression" in a query I am making using query builder in access. The problem is caused by a function inside the query.
This is the function :
Function Sudent_Selected_Next_Active_Season(Code_Student, Starting_Date As Date, Code_Project) As Boolean
Dim MyReturn
Dim SQLDate
SQLDate = Format$(Starting_Date, "\#mm\/dd\/yyyy\#")
MyReturn = DCount("[code_student]", "Project_Termination_level_2", "[Code_Student]=" & Code_Student & " and [ending] > " & SQLDate & " and [code_project] <>" & Code_Project)
If IsNull(MyReturn) Then
MyReturn = 0
End If
If MyReturn > 0 Then
Sudent_Selected_Next_Active_Season = True
Else
Sudent_Selected_Next_Active_Season = False
End If
End Function
I tried as criteria 0, -1, "0", "-1", True, False and they all give the same error.
If I remove the criteria and I review the data, all the row are composed of -1 and 0
This is the SQL used.
SELECT Sudent_Selected_Next_Active_Season([t_student].[code_student],[T_Project].[starting_date],[T_project].[code_project]) AS ActiveNextSeason
FROM (T_Project INNER JOIN T_Project_Student ON T_Project.Code_Project = T_Project_Student.Code_Project) INNER JOIN T_Student ON T_Project_Student.Code_Student = T_Student.Code_Student
WHERE (((Sudent_Selected_Next_Active_Season([t_student].[code_student],[T_Project].[starting_date],[T_project].[code_project]))=-1)) OR (((Sudent_Selected_Next_Active_Season([t_student].[code_student],[T_Project].[starting_date],[T_project].[code_project]))=-1));
If anyone could give me an idea of where to search for a solution I would appreciate it. If anyone wants more information please just ask.

Answer 1

not sure why you have the same condition twice in your WHERE clause, but in general, with those errors, you need to convert the data type on one side to match the other.
You could change the return value of your function to integer, cast the boolean return value to integer, or cast the -1 as boolean.
If you've changed the return value of the function to an integer, then the comparison with -1 should not give you a data type conversion error. Try this:
SELECT
s.code_student,p.starting_date,p.code_project,
Sudent_Selected_Next_Active_Season(s.code_student,p.starting_date,p.code_project) AS ActiveNextSeason
FROM
T_Project p INNER JOIN
T_Project_Student ps ON
p.Code_Project = ps.Code_Project INNER JOIN
T_Student s ON
ps.Code_Student = s.Code_Student
WHERE
Sudent_Selected_Next_Active_Season(s.code_student,p.starting_date,p.code_project)=-1