Seems to me your function moves the record point in the table your adding a record to. I'd suggest you use a more robust key routine that uses a lookup table of keys, or change to use GUIDs.

Jim Booth and Steve Sawyer suggested a routine that's classic in Effective techniques. There's also an example in the solutions.

OPEN DATABASE HOME() + "Samples\Tastrade\Data\tastrade"
MODIFY procedure

Look for the newid() function.

>Here is that function.
>
>

>FUNCTION NewID()
>	LPARAMETERS tlcKeyField, tlnFieldSize
>	
>	LOCAL lnNumberOfRecords, cx, i, key	 	
>	
>	IF EOF()
>		lnNumberOfRecords = RECCOUNT()
>		IF lnNumberOfRecords = 0
>			RETURN REPLICATE("A", tlnFieldSize)
>		ENDIF
>		GO lnNumberOfRecords
>		cx = &tlcKeyField
>		SKIP 1
>		
>		FOR i = LEN(cx) TO 1 STEP -1
>		 IF !SUBSTR(cx, i, 1) = 'Z'
>		   key = SUBSTR(cx, 1, i-1) ;
>		   + IIF((BETWEEN(ASC(SUBSTR(cx, i, 1)), 48, 121)), ;
>		          CHR(ASC(SUBSTR(cx, i, 1)) +1), "A") ;
>		          + REPLICATE("A", LEN(cx)-i)
>		   i = 1
>		 ENDIF
>		ENDFOR
>		
>		RETURN key
> 	ENDIF
>
>ENDFUNC
>

>
>In DefaultValue function call is: NewID("cid_child", 5)
>
>>Hi, Boban-
>>
>>>I have found out why error happens.
>>
>>Well, you've narrowed down where the source of the problem is...
>>
>>>In a ChildTable table I have field with DefaultValue which is UDF that returns value for that field.
>>>When I issue APPEND BLANK then it goes to that function to retrieve value for field and at the end of function on RETURN lcValue (value for field) error happens.
>>>
>>>When I removed UDF for DefaultValue it works, but I need that function for DefaultValue.
>>>I notice that sometimes error happens and sometimes it let me to append with my value from UDF.
>>
>>It's impossible for me to even guess, not knowing what your UDF does. You'll need to look into that code to see what it does.