Shared with synchronized

[r-const-dev]

I'm trying to implement a thread safe class, guarding data access 
with synchronized and atomicOp.

Inside the class I'm using non-shared fields, such as Nullable, 
but I guarantee the thread safety using synchronized. How can I 
tell the compiler to allow using non-shared fields/methods inside 
synchronized?

Here is my example:

import core.atomic : atomicOp;
import std.typecons : Nullable;

class SharedObject {
     private Object lock = new Object();
     private Nullable!int sharedValue;
     private int changeCount = 0;
     synchronized void modifyValue(int newValue) {
       	sharedValue = newValue;
        	atomicOp!("+=")(changeCount, 1);
     }
}

void main()
{
     shared SharedObject data = new shared SharedObject();
     data.modifyValue(3);
}

I get the error:
onlineapp.d(9): Error: template 
std.typecons.Nullable!int.Nullable.opAssign cannot deduce 
function from argument types !()(int) shared, candidates are:
/dlang/dmd/linux/bin64/../../src/phobos/std/typecons.d(2884):     
    std.typecons.Nullable!int.Nullable.opAssign()(T value)