diff --git a/src/dispnew.c b/src/dispnew.c
index 683c6ba4226..1a40939f14e 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -506,6 +506,8 @@ adjust_glyph_matrix (struct window *w, struct glyph_matrix *matrix, int x, int y
 	      row->glyphs[LEFT_MARGIN_AREA]
 		= xnrealloc (row->glyphs[LEFT_MARGIN_AREA],
 			     dim.width, sizeof (struct glyph));
+	      /* We actually need to clear only the 'frame' member, but
+                 it's easier to clear everything.  */
 	      memset (row->glyphs[LEFT_MARGIN_AREA], 0,
 		      dim.width * sizeof (struct glyph));