Checksum validation of incoming messages is not correct in some cases

private void validateCheckSum(*String* messageData) throws InvalidMessage {
        try {
            // Body length is checked at the protocol layer
            final int checksum = trailer.getInt(CheckSum.FIELD);
            if (checksum != *MessageUtils.checksum(messageData)*) {
                // message will be ignored if checksum is wrong or missing
                throw MessageUtils.newInvalidMessageException("Expected CheckSum=" + MessageUtils.checksum(messageData)
                        + ", Received CheckSum=" + checksum + " in " + messageData, this);
            }
        } catch (final FieldNotFound e) {
            throw MessageUtils.newInvalidMessageException("Field not found: " + e.field + " in " + messageData, this);
        }
    }

public static int checksum(*Charset charset*, String data, boolean isEntireMessage) {
        if (CharsetSupport.isStringEquivalent(charset)) { // optimization - skip charset encoding
            int sum = 0;
            int end = isEntireMessage ? data.lastIndexOf("\00110=") : -1;
            int len = end > -1 ? end + 1 : data.length();
            for (int i = 0; i < len; i++) {
                sum += data.charAt(i);
            }
            return sum & 0xFF; // better than sum % 256 since it avoids overflow issues
        }
        return checksum(*data.getBytes(charset)*, isEntireMessage);
    }