variable expansion bug?

I have a select DAT which selects the following row

${wTYPE}_CALLBACK

if wTYPE is set to something, it works (ie selects BUTTON_CALLBACK, FIELD_CALLBACK, etc.)

if wTYPE is unset, AND the preceding DAT’s row happens to be _CALLBACK, it should work nonetheless, but it doesn’t. In the case wTYPE is not defined it only works if I set the “select row by specified names” to _CALLBACK, but not if it’s ${wTYPE}_CALLBACK - even though IMHO it should work the same.

Is this a bug?