diff --git a/custom-prompt/custom-prompt.cc b/custom-prompt/custom-prompt.cc
index e2b45a2..6afbc18 100644
--- a/custom-prompt/custom-prompt.cc
+++ b/custom-prompt/custom-prompt.cc
@@ -159,7 +159,8 @@ void Interval::print_long(std::ostream& ostream) const
 /******************************************************************************
  * Get the ID of the currently-focused window.
  *
- * @return Active window ID.
+ * @return Active window ID. On Linux, if there is no X display running, 0 is
+ * returned. Likewise, on macOS, if no topmost window is found, 0 is returned.
  *****************************************************************************/
 extern "C" long long unsigned get_active_wid(void);
 
diff --git a/custom-prompt/get_active_wid_linux.c b/custom-prompt/get_active_wid_linux.c
index d5c77d2..d1cbb9f 100644
--- a/custom-prompt/get_active_wid_linux.c
+++ b/custom-prompt/get_active_wid_linux.c
@@ -6,6 +6,11 @@
 long long unsigned get_active_wid(void)
 {
     Display* display = XOpenDisplay(NULL);
+    if (display == NULL)
+    {
+        return 0;
+    }
+
     Window root_window = DefaultRootWindow(display);
     Atom property = XInternAtom(display, "_NET_ACTIVE_WINDOW", False);