diff --git a/src/xdisp.c b/src/xdisp.c
index f11a8aa6d55..eb00c510aaa 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -11546,6 +11546,7 @@ window_text_pixel_size (Lisp_Object window, Lisp_Object from, Lisp_Object to,
   it.bidi_p = false;
 
   int start_x;
+  ptrdiff_t start_bpos = BYTEPOS (startp);
   if (vertical_offset != 0)
     {
       int last_y;
@@ -11578,6 +11579,7 @@ window_text_pixel_size (Lisp_Object window, Lisp_Object from, Lisp_Object to,
       it.current_y = (WINDOW_TAB_LINE_HEIGHT (w)
 		      + WINDOW_HEADER_LINE_HEIGHT (w));
       start = clip_to_bounds (BEGV, IT_CHARPOS (it), ZV);
+      start_bpos = CHAR_TO_BYTE (start);
       start_y = it.current_y;
       start_x = it.current_x;
     }
@@ -11639,7 +11641,7 @@ window_text_pixel_size (Lisp_Object window, Lisp_Object from, Lisp_Object to,
   it.current_y = start_y;
   /* If FROM is on a newline, pretend that we start at the beginning
      of the next line, because the newline takes no place on display.  */
-  if (FETCH_BYTE (start) == '\n')
+  if (FETCH_BYTE (start_bpos) == '\n')
     it.current_x = 0, it.wrap_prefix_width = 0;
   if (!NILP (x_limit))
     {