diff --git a/include/wx/private/display.h b/include/wx/private/display.h
index 36f5985fbb..3f74f3dc99 100644
--- a/include/wx/private/display.h
+++ b/include/wx/private/display.h
@@ -28,10 +28,32 @@ public:
     // get it from cache.
     wxDisplayImpl* GetDisplay(unsigned n)
     {
-        if ( m_impls.empty() )
+        // Normally, m_impls should be cleared if the number of displays in the
+        // system changes because InvalidateCache() must be called. However in
+        // some ports (e.g. Mac right now, see #18318), cache invalidation never
+        // happens, so we can end up with m_impls size being out of sync with
+        // the actual number of monitors. Compensate for this here by checking
+        // if the index is invalid and invalidating the cache at least in this
+        // case.
+        //
+        // Note that this is still incorrect because we continue using outdated
+        // information if the first monitor is disconnected, for example. The
+        // only real solution is to ensure that InvalidateCache() is called,
+        // but for now this at least avoids crashes when a new display is
+        // connected.
+        if ( n >= m_impls.size() )
+        {
+            // This strange two-step resize is done to clear all the existing
+            // elements: they may not be valid any longer if the number of
+            // displays has changed.
+            m_impls.resize(0);
             m_impls.resize(GetCount());
+        }
         else if ( m_impls[n] )
+        {
+            // Just return the existing display if we have it.
             return m_impls[n];
+        }
 
         m_impls[n] = CreateDisplay(n);
         return m_impls[n];