ERROR: Non Boolean in test.

If you're writing a SynthDef
If you get this error when trying to define a SynthDef, see this page:


 * If statements in a SynthDef

More generally
This trivial example shows why this type of error occurs:

if( 'banana' ){ 12  }{  35  }

What is the correct answer? 12, or 35? 'banana' isn't a true or false value (a Boolean). (In this example it's a Symbol, since single-quote marks are used to define a Symbol in SuperCollider.)

For more info see the helpfiles "Boolean" and "Control Structures".