MzScheme returns the unique void value -- printed as #<void> -- for expressions that have undefined results in R4RS. The procedure void takes any number of arguments and returns void:
Non-global variables that are accessible but do not yet have a value are bound to the unique undefined value, printed as #<undefined>. Such variables are created by letrec-values and letrec*-values expressions (see section 3.5), partially-initialized objects (see Chapter 6), and partially-invoked units (see Chapter 7).