DSA signature verification error

        public bool verify(byte[] sig)
        {
            cs.Close();
            BigInteger r, s, w, gu1p, yu2p, gu1yu2p, u1, u2, sha, v;
            BigInteger P = new BigInteger(DSAKeyInfo.P);
            BigInteger Q = new BigInteger(DSAKeyInfo.Q);
            BigInteger G = new BigInteger(DSAKeyInfo.G);
            BigInteger Y = new BigInteger(DSAKeyInfo.Y);
            // from PuTTY:
            /*
             * Commercial SSH (2.0.13) and OpenSSH disagree over the format
             * of a DSA signature. OpenSSH is in line with the IETF drafts:
             * it uses a string "ssh-dss", followed by a 40-byte string
             * containing two 160-bit integers end-to-end. Commercial SSH
             * can't be bothered with the header bit, and considers a DSA
             * signature blob to be _just_ the 40-byte string containing
             * the two 160-bit integers. We tell them apart by measuring
             * the length: length 40 means the commercial-SSH bug, anything
             * else is assumed to be IETF-compliant.
             */
            long i = 0;
            if (sig.Length != 40)
            {
                int n = (int)((sig[i++]<<24)&0xff000000)|((sig[i++]<<16)&0x00ff0000)|
                    ((sig[i++]<<8)&0x0000ff00)|((sig[i++])&0x000000ff);
                if (n != 7 || Util.getString(sig, (int)i, 7) != "ssh-dss")
                    throw new System.Security.Cryptography.CryptographicException("Bad Data!\r\n");
                i += 7;
                n = (int)((sig[i++]<<24)&0xff000000)|((sig[i++]<<16)&0x00ff0000)|
                    ((sig[i++]<<8)&0x0000ff00)|((sig[i++])&0x000000ff);
                if (n != 40 || i+40 > sig.Length)
                    throw new System.Security.Cryptography.CryptographicException("Bad Data!\r\n");
            }
            // sig data 40 bytes (2 x 20-byte blocks)
            {
                byte[] tmp = new byte[20];
                Array.Copy(sig, i, tmp, 0, 20);
                r = new BigInteger(tmp);
                tmp = new byte[20];
                Array.Copy(sig, i+20, tmp, 0, 20);
                s = new BigInteger(tmp);
            }
            /*
             * Step 1. w <- s^-1 mod q.
             */
            w = modinv(s, Q);
            /*
             * Step 2. u1 <- SHA(message) * w mod q.
             */
            if (sha1.Hash.Length != 20)
                throw new System.Security.Cryptography.CryptographicException("Bad Data!\r\n");
            sha = new BigInteger(sha1.Hash);
            u1 = modmul(sha, w, Q);
            /*
             * Step 3. u2 <- r * w mod q.
             */
            u2 = modmul(r, w, Q);
            /*
             * Step 4. v <- (g^u1 * y^u2 mod p) mod q.
             */
            gu1p = modpow(G, u1, P);
            yu2p = modpow(Y, u2, P);
            gu1yu2p = modmul(gu1p, yu2p, P);
            v = BigInteger.Modulus(gu1yu2p, Q);//v = modmul(gu1yu2p, One, Q);
            /*
             * Step 5. v should now be equal to r.
             */
            return bignum_cmp(v, r) == 0;
        }
        private BigInteger modinv(BigInteger a, BigInteger b) { return a.ModInverse(b); }
        private BigInteger modmul(BigInteger a, BigInteger b, BigInteger n) { return BigInteger.Modulus(a * b, n); }
        private BigInteger modpow(BigInteger a, BigInteger exp, BigInteger n) { return a.ModPow(exp, n); }
        private BigInteger.Sign bignum_cmp(BigInteger a, BigInteger b) { return a.Compare(b); }
        private BigInteger bigmod(BigInteger a, BigInteger b) { return BigInteger.Modulus(a, b); }

sig

sig[i++]

if (sig.Length != 40)