diff --git a/src/ChangeLog b/src/ChangeLog
index faa2a104337305fb481beb7c46e98db35380fce0..906d17535c6ef8aef115e169fa6f326bf6c6d6e5 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,12 @@
+2012-04-23  Eli Zaretskii  <eliz@gnu.org>
+
+	* xdisp.c (pos_visible_p): If the window start position is beyond
+	ZV, start the display from buffer beginning.  Prevents assertion
+	violation in init_iterator when the minibuffer window is scrolled
+	via the scroll bar.
+
+	* window.c (window_scroll_pixel_based): Likewise.
+
 2012-04-23  Chong Yidong  <cyd@gnu.org>
 
 	* keymap.c (where_is_internal): Doc fix (Bug#10872).
diff --git a/src/window.c b/src/window.c
index af7968f9edf519545f447b8b583fbc03c4a1fb60..f93f58f9b72dd99fe6415ed667a217a491de0c41 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4221,6 +4221,11 @@ window_scroll_pixel_based (Lisp_Object window, int n, int whole, int noerror)
   void *itdata = NULL;
 
   SET_TEXT_POS_FROM_MARKER (start, w->start);
+  /* Scrolling a minibuffer window via scroll bar when the echo area
+     shows long text sometimes resets the minibuffer contents behind
+     our backs.  */
+  if (CHARPOS (start) > ZV)
+    SET_TEXT_POS (start, BEGV, BEGV_BYTE);
 
   /* If PT is not visible in WINDOW, move back one half of
      the screen.  Allow PT to be partially visible, otherwise
diff --git a/src/xdisp.c b/src/xdisp.c
index 9881adfa34d33ff221cb07e52f59653ea01b9eea..e53d3a57cd6cd2af6bc5b6b522c8dc068c7a8fbb 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -1265,6 +1265,11 @@ pos_visible_p (struct window *w, EMACS_INT charpos, int *x, int *y,
     }
 
   SET_TEXT_POS_FROM_MARKER (top, w->start);
+  /* Scrolling a minibuffer window via scroll bar when the echo area
+     shows long text sometimes resets the minibuffer contents behind
+     our backs.  */
+  if (CHARPOS (top) > ZV)
+    SET_TEXT_POS (top, BEGV, BEGV_BYTE);
 
   /* Compute exact mode line heights.  */
   if (WINDOW_WANTS_MODELINE_P (w))