==== Here is the minimized test demonstrating the bug ====
import java.lang.Bignum;
class java_lang_Bignum_isProbablePrime {
  public static void main(String args[]) {
    Bignum b=new Bignum("1");
    System.out.println(b.isProbablePrime());
  }
}
==== Here is the test output ====
java.lang.ArrayIndexOutOfBoundsException: 1
at java.lang.Bignum.isProbablePrime0(Bignum.java)
at java.lang.Bignum.isProbablePrime(Bignum.java)
at java_lang_Bignum_isProbablePrime.main(java_lang_Bignum_isProbablePrime.java:5)