diff --git a/src/window.c b/src/window.c
index cb8fe5fcdb..a7ad4d5599 100644
--- a/src/window.c
+++ b/src/window.c
@@ -2172,6 +2172,10 @@ DEFUN ("pos-visible-in-window-p", Fpos_visible_in_window_p,
 first.  POS defaults to point in WINDOW; WINDOW defaults to the
 selected window.

+Do not pass nil as POS unless you are sure that WINDOW displays the
+current buffer.  This function will emit a warning when POS is neither a
+marker, a number or t and WINDOW does not display the current buffer.
+
 If POS is visible, return t if PARTIALLY is nil; if PARTIALLY is non-nil,
 the return value is a list of 2 or 6 elements (X Y [RTOP RBOT ROWH VPOS]),
 where X and Y are the pixel coordinates relative to the top left corner
@@ -2199,10 +2203,16 @@ DEFUN ("pos-visible-in-window-p", Fpos_visible_in_window_p,
     posint = -1;
   else if (!NILP (pos))
     posint = fix_position (pos);
-  else if (w == XWINDOW (selected_window))
-    posint = PT;
   else
-    posint = marker_position (w->pointm);
+    {
+      if (XBUFFER (w->contents) != current_buffer)
+	message ("`pos-visible-in-window' called with POS nil but WINDOW's buffer is not current");
+
+      if (w == XWINDOW (selected_window))
+	posint = PT;
+      else
+	posint = marker_position (w->pointm);
+    }

   /* If position is above window start or outside buffer boundaries,
      or if window start is out of range, position is not visible.  */