I needed to implement the abstract member.
[Visual Basic]
Overrides Public Function Seek( _
   ByVal offset As Long, _
   ByVal origin As System.IO.SeekOrigin _
) As Long
[C#]
public override long Seek(
   long offset,
   SeekOrigin origin
);
See Also
InflaterInputStream Class | InflaterInputStream Members | ICSharpCode.SharpZipLib.Zip.Compression.Streams Namespace