Index: refresh.c
===================================================================
RCS file: /cvsroot/src/lib/libcurses/refresh.c,v
retrieving revision 1.112
diff -p -u -r1.112 refresh.c
--- refresh.c	24 Feb 2020 12:20:29 -0000	1.112
+++ refresh.c	8 May 2021 04:18:16 -0000
@@ -41,6 +41,7 @@ __RCSID("$NetBSD: refresh.c,v 1.112 2020
 #include <poll.h>
 #include <stdlib.h>
 #include <string.h>
+#include <assert.h>
 
 #include "curses.h"
 #include "curses_private.h"
@@ -1322,6 +1323,7 @@ makech(int wy)
 						csp->ch = (wchar_t)btowc((int)' ');
 						SET_WCOL( *csp, 1 );
 #endif /* HAVE_WCHAR */
+						assert(csp != &blank);
 						csp++;
 					}
 					return OK;
@@ -1368,7 +1370,10 @@ makech(int wy)
 			{
 				if (putch(nsp, csp, wy, wx) == ERR)
 					return ERR;
-				csp++;
+				if (!_cursesi_screen->curwin) {
+					assert(csp != &blank);
+					csp++;
+				}
 			} else {
 				putattr(nsp);
 				putattr_out(nsp);