VB Conversion to C# code error

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VB

C#5.0

Hello

Am here doing VB to C# conversion of some code but face some error listed below

Below is my VB Code

Public Overloads Function Check_GroupMembership(ByVal GroupID As Integer) As Boolean
        Dim result As Boolean = False

        If Session("user_groups") = "" Then Reset_Session_Data(Page)

        If Session("user_groups") <> "" Then
            If InStr(Session("user_groups"), "~" & GroupID & "~") <> 0 Then result = True
        End If

        Return result
    End Function

and it is my C# code after conversion

public bool Check_GroupMembership(int GroupID)
   {
       bool result = false;

       if (string.IsNullOrEmpty(Session["user_groups"]))
           Reset_Session_Data(Page);

       if (!string.IsNullOrEmpty(Session["user_groups"]))
       {
           if (String.InStr(Session["user_groups"], "~" + GroupID + "~") != 0)
               result = true;
       }

After coversion the above VB code to C# using online conversion tool following error occur:
string.IsNullOrEmpty(Session["user_groups"]):

The best overloaded method match for 'string.IsNullOrEmpty(String)' has some invalid argument.
Anyone please help me to fix it