Collection was modified, enumeration operation may not execute

I have multithreads application and i get this error

************** Exception Text **************
System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()
   at System.Collections.Generic.List`1.Enumerator.MoveNext()
   ...

I probably have problem with my collection, because on one thread i read my collection and on another thread i modify collection.

public readonly ObservableCollectionThreadSafe<GMapMarker> Markers = new ObservableCollectionThreadSafe<GMapMarker>();


public void problem()
{
  foreach (GMapMarker m in Markers)
  {
    ...
  }
}

I am trying to lock collection with this code, but doesn't work.

public void problem()
    {
       lock(Markers)
       {
         foreach (GMapMarker m in Markers)
         {
           ...
         }
       }
    }

Any ideas to fix that problem?