The binary operator GreaterThanOrEqual is not defined for the types ' Microsoft.Exchange.ExchangeSystem.ExDateTime' and 'System.DateTimeOffset

'm trying to retrieve events for a user from its Office 365 account using the REST API but filtered by the field "LastModifiedTime". For instance, retrieve all the events that will happen after the date: '2014-09-19T03:45:30.4020439Z' or '2014-09-18T04:05:00Z' but i'm always retrieving this error message:

{"error": { "code":"ErrorInternalServerError", "message":"The binary operator GreaterThanOrEqual is not defined for the types
    ' Microsoft.Exchange.ExchangeSystem.ExDateTime' and 'System.DateTimeOffset'.","innererror":{ "message":"The binary operator GreaterThanOrEqual is not defined for the types 
          'Microsoft.Exchange.ExchangeSystem.ExDateTime' and 'System.DateTimeOffset'.","type":"System.InvalidOperationException", "stacktrace":" at 
          System.Linq.Expressions.Expression.GetComparisonOperator(ExpressionType 
          binaryType, String opName, Expression left, Expression right, Boolean 
          liftToNull)\r\n at 
          System.Linq.Expressions.Expression.GreaterThanOrEqual(Expression left, 
          Expression right, Boolean liftToNull, MethodInfo method)\r\n at 
          Microsoft.Exchange.Services.OData.Model.DataEntityFilterConverter.ConvertFilterNode(QueryNode queryNode)\r\n at Microsoft.Exchange.Services.OData.Model.DataEntityFilterConverter.
          ConvertFilterClause(FilterClause filterClause)\r\n at 
          Microsoft.Exchange.Services.OData.Model.DataEntityQueryAdpater.GetEntityQueryOptions()\r\n
          at Microsoft.Exchange.Services.OData.Model.FindEventsCommand.InternalExecute()\r\n at
          Microsoft.Exchange.Services.OData.ODataCommand`2.Execute()\r\n at 
          Microsoft.Exchange.Services.OData.ODataTask.Execute(TimeSpan 
          queueAndDelayTime, TimeSpan totalTime)" 
    }
  }
}

And here are urls:

/Me/Events?$select=Subject,Start,End,LastModifiedTime&$filter=LastModifiedTime ge 2014-09-19T03:45:30.4020439Z

/Me/Events?$select=Subject,Start,End,LastModifiedTime&$filter=LastModifiedTime ge 2014-09-18T04:05:00Z

I think my error is caused by the date format but i don't know how to solve it.

Could anybody help me on that, please!

Thank you!