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.