Plank closes unexpectedly

When I try to move an icon plank closes unexpectedly.

Console Message:

WARN 09:10:39.893052] [Environment:161] XDG_SESSION_CLASS not set in this environment!
[WARN 09:10:40.003027] [Preferences:192] '/usr/share/plank/themes/Default/dock.theme' is read-only!
[WARN 09:10:40.092146] Creating surface took WAY TOO LONG (34ms), enabled downscaling for this cache!

(plank:30129): Gdk-ERROR **: 09:10:44.809: The program 'plank' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadValue (integer parameter out of range for operation)'.
  (Details: serial 4868 error_code 2 request_code 26 (core protocol) minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the GDK_SYNCHRONIZE environment
   variable to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)
Trace/breakpoint trap (core dumped)