Unable to read beyond the end of the stream error

[meatsink]

Can someone help me fix this? The full error log is

System.IO.EndOfStreamException: Unable to read beyond the end of the stream.
at System.IO.BinaryReader.ReadString()
at Celeste.BinaryPacker.FromBinary(String filename)
at Celeste.MapData.Load()
at Celeste.MapData..ctor(AreaKey area)
at Celeste.AreaData.Load()
at Celeste.GameLoader.LoadThread()
at Celeste.RunThread.RunThreadWithLogging(Action method)

(version 1.2.6.1)