Units Checker: custom qualifiers cannot be nested classes

Take this example (from https://groups.google.com/d/msg/checker-framework-discuss/M0tPOCT9RGA/1YrR_ndCDAAJ):

import org.checkerframework.checker.units.qual.*;

import org.checkerframework.framework.qual.SubtypeOf;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

public class Test {
    /**
     * Fraction
     */
    @Documented
    @Retention(RetentionPolicy.RUNTIME)
    @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
    @SubtypeOf(UnknownUnits.class)
    public static @interface frac {
        Prefix value() default Prefix.one;
    }
    
    /**
     * Percent
     */
    @Documented
    @Retention(RetentionPolicy.RUNTIME)
    @Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
    @SubtypeOf(UnknownUnits.class)
    @UnitsMultiple(quantity=frac.class, prefix=Prefix.centi)
    public static @interface pct {
    }

    public @pct Float test(final String param) {
        if (param.equals("foo")) {
            return null;
        }      
        return toFrac(3f);
    }
   
    @SuppressWarnings("units")
    @pct Float toFrac(float number) {
        return number;
    }       
}

One gets:

error: Could not load class 'Test.frac' for field 'quantity' in annotation @org.checkerframework.checker.units.qual.UnitsMultiple(quantity=Test.frac.class, prefix=org.checkerframework.checker.units.qual.Prefix.centi)
  Compilation unit: Test.java
  Last visited tree at line 35 column 5:
      public @pct Float test(final String param) {
  Exception: java.lang.ClassNotFoundException: Test.frac; Stack trace: java.net.URLClassLoader.findClass(URLClassLoader.java:381)
  java.lang.ClassLoader.loadClass(ClassLoader.java:424)
  java.lang.ClassLoader.loadClass(ClassLoader.java:357)
  java.lang.Class.forName0(Native Method)
  java.lang.Class.forName(Class.java:348)
  org.checkerframework.javacutil.AnnotationUtils.getElementValueClass(AnnotationUtils.java:662)
  org.checkerframework.checker.units.UnitsAnnotatedTypeFactory.aliasedAnnotation(UnitsAnnotatedTypeFactory.java:107)

Adding -AunitsDirs=. or -cp . has no effect.
If this is an inherent restrictions, we should document it clearly.