diff --git a/nimbus/config.nim b/nimbus/config.nim
index 1a45762ce..af85b780d 100644
--- a/nimbus/config.nim
+++ b/nimbus/config.nim
@@ -847,6 +847,13 @@ when declared(os.paramCount): # not available with `--app:lib`
     if config.net.discPort == 0:
       config.net.discPort = config.net.bindPort
 
+    if NetworkIdSet notin config.net.flags and config.net.networkId == CustomNet:
+      # WARNING: networkId and chainId are two distinct things
+      # they usage should not be mixed in other places
+      # we only set networkId to chainId if networkId not set in cli and
+      # we are using custom genesis/custom chain config via json file.
+      config.net.networkId = NetworkId(config.customGenesis.config.chainId)
+
   proc processArguments*(msg: var string): ConfigStatus =
     var opt = initOptParser()
     processArguments(msg, opt)
diff --git a/tests/test_misc.nim b/tests/test_misc.nim
index ba58327f5..5a4c1bcf0 100644
--- a/tests/test_misc.nim
+++ b/tests/test_misc.nim
@@ -28,24 +28,36 @@ proc miscMain*() =
     const genesisFile = "tests" / "customgenesis" / "calaveras.json"
     test "networkid cli":
       var msg: string
-      var opt = initOptParser("--customnetwork:" & genesisFile & " --networkid:123")
+      var opt = initOptParser("--customnetwork:" & genesisFile & " --networkid:345")
       let res = processArguments(msg, opt)
       if res != Success:
         echo msg
         quit(QuitFailure)
 
       let conf = getConfiguration()
-      check conf.net.networkId == 123.NetworkId
+      check conf.net.networkId == 345.NetworkId
 
-    test "networkid first, customnetwork next":
+    test "networkid first, customnetwork next":      
       var msg: string
-      var opt = initOptParser(" --networkid:123 --customnetwork:" & genesisFile)
+      var opt = initOptParser("--networkid:678 --customnetwork:" & genesisFile)
       let res = processArguments(msg, opt)
       if res != Success:
         echo msg
         quit(QuitFailure)
 
       let conf = getConfiguration()
+      check conf.net.networkId == 678.NetworkId
+
+    test "networkid not set, copy from chainId of customnetwork":      
+      let conf = getConfiguration()
+      conf.net.flags.excl NetworkIdSet      
+      var msg: string
+      var opt = initOptParser("--customnetwork:" & genesisFile)
+      let res = processArguments(msg, opt)
+      if res != Success:
+        echo msg
+        quit(QuitFailure)
+      
       check conf.net.networkId == 123.NetworkId
 
 when isMainModule: