Why malformed is 1 not 2 in GB18030?

In Java GB18030 charset, decode impl have this code

// Either 2 or 4 byte sequence follows                        if ( sl - sp < 2 )                            return CoderResult.UNDERFLOW;                        byte2 = sa[sp + 1] & 0xFF;                        inputSize = 2;                        if (byte2 < 0x30)                                return CoderResult.malformedForLength(1);

This code position is in GB18030.Decode#decodeArrayLoop, so when the char is double byte or four bytes to show, I don't know why the second byte (namely byte2) if malformed will throw malformed exception with 1 not 2.

Maybe it should be thrown exception with 2 ?