It's impossible in VFP to create a variable that doesn't have data type assigned to it. If data type is not specified the Logical data type is used as default.

zz = Null
? TYPE("zz")

>This behavior is due to the fact that your variable never had a data type assigned to it while hte fields and scattered variables doi have a data type assigned to them. Try this one;
>
>

>zz = 0
>zz = NULL
>?str(zz)
>

>
>No error because the data type of zz has been set to numeric so the STR() function works.