Using reflection to feed Collection in DbEntityEntry

for some reason I'm trying to dynamically count the number of item of a related collection of an entity.

Dynamically write the equivalent of

Int32 i = Entry(client).Collection(x => x.Contacts).Query().Count();

The dynamic part being on x => x.Contacts. So:

Client client = Clients.First();
Type t = typeof(Client);
var param = Expression.Parameter(t, "x" );
PropertyInfo pi = t.GetProperty("Contacts");
MemberExpression expr = Expression.Property(param, pi);
var exp = Expression.Lambda(expr, param);

from here the following runs

Int32 i = Entry(client).Collection(
    (Expression<Func<Cclb.Domain.Entities.Client, ICollection<Cclb.Domain.Entities.ClientContact>>>)
     exp).Query().Count();

but not

//same without the cast
Int32 i = Entry(client).Collection(exp).Query().Count();

it raises:

 Impossible to convert from 'System.Linq.Expressions.LambdaExpression' to 'string'

That is, it does not call the 'good' method. Is it possible to have it runs without the cast ?