You could use a <<link>> macro to ensure that the variable has a proper value before allowing the player to continue. For example:
What's your favorite pie?
* <<radiobutton "_pie" "blueberry">> Blueberry?
* <<radiobutton "_pie" "cherry">> Cherry?
* <<radiobutton "_pie" "coconut cream">> Coconut cream?
<<link "Continue">>
<<if def _pie>>
<<set $pie to _pie>>
<<goto [[Next Passage]]>>
<</if>>
<</link>>
What exactly you test for in the <<if>> condition would depend on what exactly you're doing. In the above example we ensure that the temporary variable _pie has a value before assigning that value to a story variable and sending the player to the next passage.
Here's a slightly more complicated example which adds an error message for the player:
What's your favorite pie?
* <<radiobutton "_pie" "blueberry">> Blueberry?
* <<radiobutton "_pie" "cherry">> Cherry?
* <<radiobutton "_pie" "coconut cream">> Coconut cream?
<<link "Continue">>
<<if def _pie>>
<<set $pie to _pie>>
<<goto [[Next Passage]]>>
<<else>>
<<replace "#error-pie">> — Please select a pie.<</replace>>
<</if>>
<</link>>@@#error-pie;@@