diff --git a/README.md b/README.md
index 447976c4..fe365cb0 100644
--- a/README.md
+++ b/README.md
@@ -67,6 +67,7 @@ The following dependencies are needed by the examples and test programs:
                  cause an uncaught `BadWindow` error
  - [X11] Bugfix: No check was made for the presence GLX 1.3 when
                  `GLX_SGIX_fbconfig` was unavailable
+ - [X11] Bugfix: The message type of ICCCM protocol events was not checked
 
 
 ## Contact
diff --git a/src/x11_init.c b/src/x11_init.c
index 18d92fdd..4dc46d48 100644
--- a/src/x11_init.c
+++ b/src/x11_init.c
@@ -430,6 +430,9 @@ static GLboolean initExtensions(void)
     Bool supported;
 
     // Find or create window manager atoms
+    _glfw.x11.WM_PROTOCOLS = XInternAtom(_glfw.x11.display,
+                                         "WM_PROTOCOLS",
+                                         False);
     _glfw.x11.WM_STATE = XInternAtom(_glfw.x11.display, "WM_STATE", False);
     _glfw.x11.WM_DELETE_WINDOW = XInternAtom(_glfw.x11.display,
                                              "WM_DELETE_WINDOW",
diff --git a/src/x11_platform.h b/src/x11_platform.h
index 25a54612..f0dc6081 100644
--- a/src/x11_platform.h
+++ b/src/x11_platform.h
@@ -107,6 +107,7 @@ typedef struct _GLFWlibraryX11
     XContext        context;
 
     // Window manager atoms
+    Atom            WM_PROTOCOLS;
     Atom            WM_STATE;
     Atom            WM_DELETE_WINDOW;
     Atom            NET_WM_NAME;
diff --git a/src/x11_window.c b/src/x11_window.c
index 9042cc6f..eeb82474 100644
--- a/src/x11_window.c
+++ b/src/x11_window.c
@@ -739,25 +739,32 @@ static void processEvent(XEvent *event)
         {
             // Custom client message, probably from the window manager
 
-            if ((Atom) event->xclient.data.l[0] == _glfw.x11.WM_DELETE_WINDOW)
-            {
-                // The window manager was asked to close the window, for example by
-                // the user pressing a 'close' window decoration button
+            if (event->xclient.message_type == None)
+                break;
 
-                _glfwInputWindowCloseRequest(window);
-            }
-            else if (_glfw.x11.NET_WM_PING &&
-                     (Atom) event->xclient.data.l[0] == _glfw.x11.NET_WM_PING)
+            if (event->xclient.message_type == _glfw.x11.WM_PROTOCOLS)
             {
-                // The window manager is pinging the application to ensure it's
-                // still responding to events
+                if (_glfw.x11.WM_DELETE_WINDOW &&
+                    (Atom) event->xclient.data.l[0] == _glfw.x11.WM_DELETE_WINDOW)
+                {
+                    // The window manager was asked to close the window, for example by
+                    // the user pressing a 'close' window decoration button
 
-                event->xclient.window = _glfw.x11.root;
-                XSendEvent(_glfw.x11.display,
-                           event->xclient.window,
-                           False,
-                           SubstructureNotifyMask | SubstructureRedirectMask,
-                           event);
+                    _glfwInputWindowCloseRequest(window);
+                }
+                else if (_glfw.x11.NET_WM_PING &&
+                        (Atom) event->xclient.data.l[0] == _glfw.x11.NET_WM_PING)
+                {
+                    // The window manager is pinging the application to ensure it's
+                    // still responding to events
+
+                    event->xclient.window = _glfw.x11.root;
+                    XSendEvent(_glfw.x11.display,
+                            event->xclient.window,
+                            False,
+                            SubstructureNotifyMask | SubstructureRedirectMask,
+                            event);
+                }
             }
             else if (event->xclient.message_type == _glfw.x11.XdndEnter)
             {