DateTime.ParseExact gives String was not recognized as a valid DateTime.

I'm trying to parse a date string into a DateTime variable. I've found out that ParseExact is the way to do it, but I try this I get the error:

String was not recognized as a valid DateTime.

string timeFormat = "dd-MM-yyyy hh:mm:ss";
DateTime startDate = DateTime.ParseExact(reader["startdate"].ToString(), timeFormat, CultureInfo.InvariantCulture);
DateTime nextDate = DateTime.ParseExact(reader["nextdate"].ToString(), timeFormat, null);

I've tried both with null (which happens to work on another page), and the CultureInfo.InvariantCulture.

reader["startdate"].ToString() output: 01-08-2012 15:39:09

and

reader["nextdate"].ToString() output: 01-08-2012 15:39:09

I think it should work, but it doesn't.

Somebody have an idea what is wrong? :)