* frame.c (store_frame_param): Fix error message punctuation.

src/frame.c

@@ -1062,7 +1062,7 @@ store_frame_param (f, prop, val)
 	error ("Surrogate minibuffer windows must be minibuffer windows.");
 
       if (FRAME_HAS_MINIBUF_P (f) || FRAME_MINIBUF_ONLY_P (f))
-	error ("Can't change the surrogate minibuffer of a frame with its own minibuffer.");
+	error ("can't change the surrogate minibuffer of a frame with its own minibuffer");
 
       /* Install the chosen minibuffer window, with proper buffer.  */
       f->minibuffer_window = val;