Assigning values to dictionary elements

[!NoItAll]

I've created a dictionary of a simple structure

Private Enum ClientStatus
    Alive = 0
    Zombie = 1
    Corpse = 2
End Enum

Private Structure ClientHeartbeat_type
   Dim Timestamp as DateTime
   Dim Status as ClientStatus
End Structure

Private ClientList as Dictionary(of String, ClientHeartbeat_type)

The intent is that the KEY for the dictionary when I create an element will be the IPV4 IP Address of the client.
So I add an element to the dictionary as follows

Dim MyClient as new ClientHeartbeat_type with {.status = Clientstatus.Zombie, .Timestamp = Now}
ClientList.add(ClientIP, MyClient)

Later in my code I want to change values to the structure in the dictionary like this:

ClientList(ClientIPAddress).Status = ClientStatus.Alive

The problem is that it complains: "The expression is a value and therefore cannot be the target of an assignment"

But this works:

ClientList(ClientIP) = New ClientHeartbeat_type with {.status = Clientstatus.Alive, .Timestamp = Now}

So I am updating the entire structure. Why won't it let me update just one element of the structure.