diff --git a/extensions/bytecoin-v2/cryptonight-v7.json b/extensions/bytecoin-v2/cryptonight-v7.json
index f8335dd..ffb994e 100644
--- a/extensions/bytecoin-v2/cryptonight-v7.json
+++ b/extensions/bytecoin-v2/cryptonight-v7.json
@@ -112,7 +112,7 @@
           "parameters": {
             "text": [
               "  uint32_t cn_variant = 0;",
-              "  if (m_POWCryptoNightV7BlockIndex >= block.getBlockIndex() && m_POWCryptoNightV7LastBlock <= block.getBlockIndex())",
+              "  if (m_POWCryptoNightV7BlockIndex && m_POWCryptoNightV7BlockIndex <= block.getBlockIndex() && (m_POWCryptoNightV7LastBlock == 0 || m_POWCryptoNightV7LastBlock >= block.getBlockIndex()))",
               "    cn_variant = 1;",
               "  return check_hash(block.getBlockLongHash(context, cn_variant), currentDifficulty);"
             ]
@@ -124,7 +124,7 @@
           "parameters": {
             "text": [
               "  uint32_t cn_variant = 0;",
-              "  if (m_POWCryptoNightV7BlockIndex >= cachedBlock.getBlockIndex() && m_POWCryptoNightV7LastBlock <= cachedBlock.getBlockIndex())",
+              "  if (m_POWCryptoNightV7BlockIndex && m_POWCryptoNightV7BlockIndex <= cachedBlock.getBlockIndex() && (m_POWCryptoNightV7LastBlock == 0 || m_POWCryptoNightV7LastBlock >= cachedBlock.getBlockIndex()))",
               "    cn_variant = 1;",
               "  if (!check_hash(cachedBlock.getBlockLongHash(context, cn_variant), currentDifficulty)) {"
             ]
@@ -141,7 +141,7 @@
           "parameters": {
             "text": [
               "      uint32_t cn_variant = 0;",
-              "      if (m_currency.POWCryptoNightV7BlockIndex() >= cb.getBlockIndex() && m_currency.POWCryptoNightV7LastBlock() <= cb.getBlockIndex())",
+              "      if (m_currency.POWCryptoNightV7BlockIndex() && m_currency.POWCryptoNightV7BlockIndex() <= cb.getBlockIndex() && (m_currency.POWCryptoNightV7LastBlock() == 0 || m_currency.POWCryptoNightV7LastBlock() >= cb.getBlockIndex()))",
               "        cn_variant = 1;"
             ]
           }
@@ -161,7 +161,7 @@
           "parameters": {
             "text": [
               "            uint32_t cn_variant = 0;",
-              "//            if (m_currency.POWCryptoNightV7BlockIndex() >= cb.getBlockIndex() && m_currency.POWCryptoNightV7LastBlock() <= cb.getBlockIndex())",
+              "//            if (m_currency.POWCryptoNightV7BlockIndex() && m_currency.POWCryptoNightV7BlockIndex() <= cb.getBlockIndex() && (m_currency.POWCryptoNightV7LastBlock() == 0 || m_currency.POWCryptoNightV7LastBlock() >= cb.getBlockIndex()))",
               "//              cn_variant = 1;"
             ]
           }
@@ -172,7 +172,7 @@
           "parameters": {
             "text": [
               "        uint32_t cn_variant = 0;",
-              "//        if (m_currency.POWCryptoNightV7BlockIndex() >= cb.getBlockIndex() && m_currency.POWCryptoNightV7LastBlock() <= cb.getBlockIndex())",
+              "//        if (m_currency.POWCryptoNightV7BlockIndex() && m_currency.POWCryptoNightV7BlockIndex() <= cb.getBlockIndex() && (m_currency.POWCryptoNightV7LastBlock() == 0 || m_currency.POWCryptoNightV7LastBlock() >= cb.getBlockIndex()))",
               "//          cn_variant = 1;"
             ]
           }