diff --git a/toplevel.h b/toplevel.h
--- a/toplevel.h
+++ b/toplevel.h
@@ -737,11 +737,6 @@
     return desktop() == d || /*desk == 0 ||*/ isOnAllDesktops();
 }
 
-inline bool Toplevel::isOnActivity(const QString &activity) const
-{
-    return activities().isEmpty() || activities().contains(activity);
-}
-
 inline bool Toplevel::isOnCurrentDesktop() const
 {
     return isOnDesktop(VirtualDesktopManager::self()->current());
diff --git a/toplevel.cpp b/toplevel.cpp
--- a/toplevel.cpp
+++ b/toplevel.cpp
@@ -402,6 +402,27 @@
 #endif
 }
 
+bool Toplevel::isOnActivity(const QString &activity) const
+{
+    // Checking whether we are on an invalid, that is non-existing activity
+    if (Activities::self()) {
+        const auto &existingActivities = Activities::self()->all();
+        const auto &clientActivities = activities();
+        const bool atLeastOneValidActivity =
+            std::find_if(
+                    clientActivities.cbegin(), clientActivities.cend(),
+                    [&existingActivities] (const QString &clientActivity) {
+                        return existingActivities.contains(clientActivity);
+                    }) != clientActivities.cend();
+
+            if (!atLeastOneValidActivity) {
+                return true;
+            }
+    }
+
+    return activities().isEmpty() || activities().contains(activity);
+}
+
 void Toplevel::elevate(bool elevate)
 {
     if (!effectWindow()) {