diff --git a/lazarus/DBConnection.pas b/lazarus/DBConnection.pas
index c64831a..b09f14e 100644
--- a/lazarus/DBConnection.pas
+++ b/lazarus/DBConnection.pas
@@ -6,7 +6,7 @@ interface
 
 uses
   Classes, SysUtils, sqlite3conn, sqldb, DB, FileUtil, Forms, Controls,
-  Graphics, Dialogs, DBGrids, book, booktype, rental, student;
+  Graphics, Dialogs, DBGrids, book, booktype, rental, student, DBConstants;
 
 type
   ArrayOfStudents = array of TStudent;
@@ -423,7 +423,7 @@ function TDBConnection.updateInsertStudent(student: TStudent): boolean;
       FieldByName('first_name').AsString := student.getFirstName;
       FieldByName('class_name').AsString := student.getClassName;
 
-      if student.getBirth <= -1 then //if set to null
+      if student.getBirth = SQLNull then //if set to null
         FieldByName('birth').Clear
       else
         FieldByName('birth').AsDateTime := student.getBirth;
@@ -590,7 +590,7 @@ function TDBConnection.updateInsertRental(rental: TRental): boolean;
       FieldByName('book_id').AsLargeInt := rental.getBookId;
       FieldByName('rental_date').AsDateTime := rental.getRentalDate;
 
-      if rental.getReturnDate <= -1 then //if set to null
+      if rental.getReturnDate = SQLNull then //if set to null
         FieldByName('return_date').Clear
       else
         FieldByName('return_date').AsDateTime := rental.getReturnDate;
diff --git a/lazarus/dbconstants.pas b/lazarus/dbconstants.pas
new file mode 100644
index 0000000..7bbe344
--- /dev/null
+++ b/lazarus/dbconstants.pas
@@ -0,0 +1,12 @@
+unit DBConstants;
+
+{$mode objfpc}{$H+}
+
+interface
+
+const
+  SQLNull = -1;
+
+implementation
+
+end.
diff --git a/lazarus/rental.pas b/lazarus/rental.pas
index aa4f1b5..02d0c4a 100644
--- a/lazarus/rental.pas
+++ b/lazarus/rental.pas
@@ -5,7 +5,7 @@
 interface
 
 uses
-  Classes, SysUtils;
+  Classes, SysUtils, DBConstants;
 
 type
   TRental = class
@@ -146,8 +146,8 @@ constructor TRental.Create;
   self.id := -1;
   self.book_id := -1;
   self.student_id := -1;
-  self.return_date := -1; // -1 or smaller equals NULL
-  self.rental_date := -1;
+  self.return_date := SQLNull;
+  self.rental_date := SQLNull;
 end;
 
 end.
diff --git a/lazarus/student.pas b/lazarus/student.pas
index bf0f697..c5a3443 100644
--- a/lazarus/student.pas
+++ b/lazarus/student.pas
@@ -5,7 +5,7 @@
 interface
 
 uses
-  Classes, SysUtils;
+  Classes, SysUtils, DBConstants;
 
 type
   TStudent = class
@@ -172,7 +172,7 @@ constructor TStudent.Create;
   self.last_name := '';
   self.first_name := '';
   self.class_name := '';
-  self.birth := -1; // -1 or smaller equals NULL
+  self.birth := SQLNull;
   self.ldap_user := '';
 end;