StringBuffer.insert() compiles with CharSequence with 1.4 target

FULL PRODUCT VERSION :
java version "1.6.0_01"
Java(TM) SE Runtime Environment (build 1.6.0_01-b06)
Java HotSpot(TM) Client VM (build 1.6.0_01-b06, mixed mode, sharing)


ADDITIONAL OS VERSION INFORMATION :
Linux vorlon.ice 2.6.14-1.1644_FC4 #1 Sun Nov 27 03:25:11 EST 2005 i686 i686 i386 GNU/Linux

A DESCRIPTION OF THE PROBLEM :

When StringBuffer.insert(0, StringBuffer buf) is compiled on JDK 1.6 or JDK 1.5 with a JDK 1.4 target,
the following exception is thrown:

Exception in thread "main" java.lang.NoSuchMethodError: java.lang.StringBuffer.insert(ILjava/lang/CharSequence;)Ljava/lang/StringBuffer;
at InsertTest.main(InsertTest.java:6)


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
public class InsertTest {
    public static void main(String[] args) {
        StringBuffer buf = new StringBuffer("hello");
        buf.insert(0, buf);
        System.out.println(buf);
    }
}

/usr/local/jdk1.6.0_01/bin/javac -target 1.4 -source 1.4 InsertTest.java
/usr/local/java/bin/java InsertTest

Exception in thread "main" java.lang.NoSuchMethodError: java.lang.StringBuffer.insert(ILjava/lang/CharSequence;)Ljava/lang/StringBuffer;
at InsertTest.main(InsertTest.java:6)



EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The code should execute with no exceptions.


ACTUAL -
Exception in thread "main" java.lang.NoSuchMethodError: java.lang.StringBuffer.insert(ILjava/lang/CharSequence;)Ljava/lang/StringBuffer;
at InsertTest.main(InsertTest.java:6)



ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "main" java.lang.NoSuchMethodError: java.lang.StringBuffer.insert(ILjava/lang/CharSequence;)Ljava/lang/StringBuffer;
at InsertTest.main(InsertTest.java:6)



REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
public class InsertTest {
    public static void main(String[] args) {
        StringBuffer buf = new StringBuffer("hello");
        buf.insert(0, buf);
        System.out.println(buf);
    }
}


---------- END SOURCE ----------