diff --git a/src/textual/app.py b/src/textual/app.py
index fcbb0b729d..538a5f5193 100644
--- a/src/textual/app.py
+++ b/src/textual/app.py
@@ -1991,7 +1991,7 @@ def install_screen(self, screen: Screen, name: str) -> None:
             raise ScreenError(f"Can't install screen; {name!r} is already installed")
         if screen in self._installed_screens.values():
             raise ScreenError(
-                "Can't install screen; {screen!r} has already been installed"
+                f"Can't install screen; {screen!r} has already been installed"
             )
         self._installed_screens[name] = screen
         self.log.system(f"{screen} INSTALLED name={name!r}")