Msg 16943, Level 16, State 4, Line 27: Could not complete cursor operation because the table schema changed after the cursor was declared.

IF EXISTS (SELECT * FROM sysobjects WHERE xtype = 'U' AND name = 'AKG111')
BEGIN
    DROP TABLE AKG111;
    PRINT 'Table, AKG111 is dropped';
END
GO

IF EXISTS (SELECT * FROM sysobjects WHERE xtype = 'U' AND name = 'AKG222')
BEGIN
    DROP TABLE AKG222;
    PRINT 'Table, AKG222 is dropped';
END
GO

BEGIN
   CREATE TABLE AKG111 ( NAME NVARCHAR(100) NOT NULL , TYPE NVARCHAR(100) NOT NULL);
   PRINT 'Table, AKG111 is Created';

   CREATE TABLE AKG222 ( NAME NVARCHAR(100) NOT NULL , TYPE NVARCHAR(100) NOT NULL);
   PRINT 'Table, AKG222 is Created';

   ALTER TABLE AKG111 ADD CONSTRAINT AKG111_PK PRIMARY KEY (NAME, TYPE) WITH (FILLFACTOR = 80, PAD_INDEX = ON);
   PRINT 'Created PK, AKG111_PK on Table, AKG111';

   ALTER TABLE AKG222 ADD CONSTRAINT AKG222_PK PRIMARY KEY (NAME, TYPE) WITH (FILLFACTOR = 80, PAD_INDEX = ON);
   PRINT 'Created PK, AKG222_PK on Table, AKG222';
END
GO

BEGIN
    DECLARE @parent_table VARCHAR(80), @cons_name VARCHAR(80);

    DECLARE drop_prmy_keys
     CURSOR
        FOR
     SELECT s2.name
           ,s1.name
       FROM sys.objects          s1
           ,sys.objects          s2
      WHERE s1.type              = 'PK'
        AND s1.parent_object_id  = s2.object_id
        AND s1.name              NOT IN (SELECT NAME FROM AKG111 WHERE TYPE = 'constraint')
        AND s2.name              IN ('AKG111','AKG222')
   ORDER BY s2.name;

   OPEN drop_prmy_keys;
   FETCH NEXT FROM drop_prmy_keys INTO @parent_table, @cons_name;

   WHILE(@@FETCH_STATUS=0)
   BEGIN
      PRINT 'Alter Table ' + @parent_table + ' To Drop Constraint ' + @cons_name;
      EXEC('ALTER TABLE ' + @parent_table + ' DROP CONSTRAINT ' + @cons_name);
      PRINT 'Constraint,  '+ @cons_name +  ' on Table ' + @parent_table + ' is Dropped.' ;

      FETCH NEXT FROM drop_prmy_keys INTO @parent_table, @cons_name;
   END

   CLOSE drop_prmy_keys;
   DEALLOCATE drop_prmy_keys;
END
GO

The above code fails with the error mentioned in the subject line. However if I comment the NOT IN condition in the cursor declaration it executes successfully. 

It leads to the conclusion that a "table being altered in the cursor loop can not be used in the cursor declaration" or a "table used in the cursor declaration can not be altered in the cursor loop iteration".

However I could not find any microsoft mssql documentation stating this restriction.

Can anyone please explain this behavior and any pointer to any documentation which mentions about this restriction?