Problems with XF86MiscSetMouseSettings

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I've been having issues getting XF86MiscSetMouseSettings() to work. 
I've put together a really simple test case (attached) that results in 
the following error:

X Error of failed request:  139
   Major opcode of failed request:  136 (XFree86-Misc)
   Minor opcode of failed request:  5 (XF86MiscSetMouseSettings)
   Value in failed request:  0x3a
   Serial number of failed request:  9
   Current serial number in output stream:  10

Could somebody tell me what's wrong with my test case, or if this is a 
bug in the xf86misc extension?  I'd really appreciate it.

Thanks,

Michael

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#include <X11/Xlib.h>
#include <X11/extensions/xf86misc.h>

int main (int argc, char *argv[]) {
  Display* dpy = XOpenDisplay(":0.0");
  XF86MiscMouseSettings mouse_settings;

  XF86MiscGetMouseSettings(dpy, &mouse_settings);
  XF86MiscGetMouseSettings(dpy, &mouse_settings);
  XF86MiscGetMouseSettings(dpy, &mouse_settings);
//   XF86MiscSetMouseSettings(dpy, &mouse_settings);

  // Any X call here appears to cause an error
  XF86MiscGetMouseSettings(dpy, &mouse_settings);
}

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test: testmouse.c
	gcc -o testmouse testmouse.c -L/usr/X11R6/lib -lXxf86misc -lXext -lX11

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