Cannot cast from source type to destination type.

public void AddTickData(List<Dictionary<string, string>> ticks)
{
  if (!data.ContainsKey("tickData"))
  {
    data.Add("tickData", ticks);
  }
  else
  {
    // Got an exception here:
    List<Dictionary<string, string>> previousTicks = (List<Dictionary<string, string>>)data ["tickData"];
    // etc.
  }
}