I passed a non DECLARED kind to Types.directSupertypes(TypeMirror t) , and it returned a List of Types.
List contains one null element.

Expected a list with 0 elements or 'IllegalArgumentException'.

bash-3.00$ cat CaseAP.java
import java.util.*;
import javax.annotation.processing.*;
import javax.lang.model.element.*;
import javax.lang.model.type.*;
import javax.lang.model.util.*;
import static javax.lang.model.SourceVersion.*;
import static javax.lang.model.type.TypeKind.*;

@SupportedAnnotationTypes("*")
@SupportedSourceVersion(RELEASE_6)
@SupportedOptions("-verbose")
public class CaseAP extends AbstractProcessor {
    public void init(ProcessingEnvironment penv) {
        super.init(penv);
    }
    public boolean process(Set<? extends TypeElement> typeElementSet,
            RoundEnvironment renv) {
        Vector<TypeMirror> listArrayElementList = new Vector<TypeMirror>();
        for( TypeElement element : renv.getSpecifiedTypeElements()) {
            List<? extends Element > list = element.getEnclosedElements();
            for( Element e : list) {
                listArrayElementList.add(e.asType());
            }
            listArrayElementList.add(element.asType());
        }
        System.out.println(" Identified TypeMirror count "+listArrayElementList.size());
        for(TypeMirror mirror : listArrayElementList) {
            System.out.println(" TypeKind("+mirror.toString()+") = "+mirror.getKind());
            List <? extends TypeMirror> list = processingEnv.getTypeUtils().directSupertypes(mirror);
            System.out.println(" size(types.directSupertypes("+mirror.toString()+")) = "+list.size());
            for(TypeMirror tm : list) {
                if ( tm != null ) {
                    System.out.println(" ->> superType("+mirror.toString()+") = "+tm.toString());
                } else {
                    System.out.println(" **-->>got a null superType for "+mirror.toString());
                }
            }
        }
        return true ;
    }
}
bash-3.00$ cat Case1.java
public class Case1 {
    private int i;
    private Float flt;
    public void doMethod2_1(String message){
        System.out.println(" message "+message);
    }
}
bash-3.00$ $JAVA_HOME/bin/javac -classpath $CLASSPATH -processor CaseAP Case1.java
bash-3.00$ ksh compile.sh
     Identified TypeMirror count 5
  TypeKind(()void) = EXECUTABLE
  size(types.directSupertypes(()void)) = 1
  **-->>got a null superType for ()void
  TypeKind(int) = INT
  size(types.directSupertypes(int)) = 1
  **-->>got a null superType for int
  TypeKind(java.lang.Float) = DECLARED
  size(types.directSupertypes(java.lang.Float)) = 2
  ->> superType(java.lang.Float) = java.lang.Number
  ->> superType(java.lang.Float) = java.lang.Comparable<java.lang.Float>
  TypeKind((java.lang.String)void) = EXECUTABLE
  size(types.directSupertypes((java.lang.String)void)) = 1
  **-->>got a null superType for (java.lang.String)void
  TypeKind(Case1) = DECLARED
  size(types.directSupertypes(Case1)) = 1
  ->> superType(Case1) = java.lang.Object
     Identified TypeMirror count 0
bash-3.00$ $JAVA_HOME/bin/java -version
java version "1.6.0-auto"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0-auto-292b)
Java HotSpot(TM) Server VM (build 1.6.0-ea-b56, mixed mode)