CharsetEncoder.canEncode(ch) != CharsetEncoder.canEncode("" + ch) for some Charsets?

I ran this simple test with JDK 5 and JDK 6 on Mac OSx and Windows XP,

SortedMap<String, Charset> map = Charset.availableCharsets();
        for (Charset cs : map.values()) {
            try {
                CharsetEncoder encoder = cs.newEncoder();

                char ch = '\u5185';
                if (encoder.canEncode(ch) != encoder.canEncode("" + ch))
                    System.out.println("Encoder: " + cs.name() + " failed");
            } catch (Exception e) {
                //System.out.println("Encoder: " + cs.name() + " Error");
            }
        }

It failed
JDK 5 (1.5.0_15) windows xp:
Encoder: ISO-2022-KR failed
Encoder: x-IBM1381 failed
Encoder: x-IBM1383 failed
Encoder: x-IBM942 failed
Encoder: x-IBM942C failed
Encoder: x-IBM943 failed
Encoder: x-IBM943C failed

JDK 6 (1.6.0_04) Windows xp:
Encoder: x-ISO-2022-CN-CNS failed

Mac Osx (JDK 5 1.5.0_13)
Encoder: MacRoman failed
Encoder: x-IBM1381 failed
Encoder: x-IBM1383 failed
Encoder: x-IBM942 failed
Encoder: x-IBM942C failed
Encoder: x-IBM943 failed
Encoder: x-IBM943C failed