Unable to create network. Invalid network parameter: Subnet mask cannot be empty.

I am facing a problem while adding an “isolated” network to an existing vApp. Getting exception thrown with message “Unable to create network “mynetwork”. Invalid network parameter: Subnet mask cannot be empty.”

Below is the code,

VAppNetworkConfigurationType vAppNetworkConfigurationType = new VAppNetworkConfigurationType();
 vAppNetworkConfigurationType.networkName = “myNetwork”;
vAppNetworkConfigurationType.Configuration = new NetworkConfigurationType();
 vAppNetworkConfigurationType.Configuration.FenceMode = “isolated”;
vAppNetworkConfigurationType.Configuration.IpScope = new IpScopeType();
 vAppNetworkConfigurationType.Configuration.IpScope.Gateway = “192.168.2.1″;
 vAppNetworkConfigurationType.Configuration.IpScope.Netmask = “255.255.255.0″;
 IpRangeType ipRange = new IpRangeType();
 ipRange.StartAddress = “192.168.2.100″;
 ipRange.EndAddress = “192.168.2.199″;
 vAppNetworkConfigurationType.Configuration.IpScope.IpRanges = new IpRangesType();
 vAppNetworkConfigurationType.Configuration.IpScope.IpRanges.IpRange = new IpRangeType[]{ipRange};
 NetworkConfigSectionType networkConfigSectionType = new NetworkConfigSectionType();
 networkConfigSectionType.NetworkConfig = new VAppNetworkConfigurationType[] {vAppNetworkConfigurationType};
 networkConfigSectionType.Info = new Msg_Type();
 Task task_net = vApp.UpdateSection(networkConfigSectionType);
 task_net.WaitForTask(timeout); <<< Exception is thrown in this line