Any expression with a NULL will result as NULL.
Ex.: ? "Test" + NULL && Print NULL

Here is what you should do:
lcSQLString = "INSERT INTO DatabaseName..TableName(field1,field2) VALUES("+;
IIF(ISNULL(MyCursor.field1),"NULL","'"+ALLTRIM(MyCursor.field1)+"'")+","+;
IIF(ISNULL(MyCursor.field2),"NULL",ALLTRIM(STR(MyCursor.field2)))+")"

Notice the "" around the NULL.

HTH

>I have the following statement in my .prg, which keeps on producing the error message "Function argument value, type, or count is invalid":
>
>lcSQLString = "INSERT INTO DatabaseName..TableName(field1,field2) VALUES("+;
>IIF(ISNULL(MyCursor.field1),NULL,"'"+ALLTRIM(MyCursor.field1)+"'")+","+;
>IIF(ISNULL(MyCursor.field2),NULL,ALLTRIM(STR(MyCursor.field2)))+")"
>
>where:
>field1 - character
>field2 - numeric
>
>When I turn debugger on to check the value of lcSQLString, it equals to .NULL.
>
>Does anyone have an explanation for this? Or a better way?