diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 7e730be..210e21f 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -4412,12 +4412,17 @@
 		// Save flags for recursion.  They can change for e.g.
 		// :s/^/\=execute("s#^##gn")
 		subflags_save = subflags;
+
+		// Disallow changing text or switching window in an expression.
+		++textwinlock;
 #endif
 		// get length of substitution part
 		sublen = vim_regsub_multi(&regmatch,
 				    sub_firstlnum - regmatch.startpos[0].lnum,
 			       sub, sub_firstline, FALSE, magic_isset(), TRUE);
 #ifdef FEAT_EVAL
+		--textwinlock;
+
 		// If getting the substitute string caused an error, don't do
 		// the replacement.
 		// Don't keep flags set by a recursive call.
@@ -4518,9 +4523,15 @@
 		mch_memmove(new_end, sub_firstline + copycol, (size_t)copy_len);
 		new_end += copy_len;
 
+#ifdef FEAT_EVAL
+		++textwinlock;
+#endif
 		(void)vim_regsub_multi(&regmatch,
 				    sub_firstlnum - regmatch.startpos[0].lnum,
 				      sub, new_end, TRUE, magic_isset(), TRUE);
+#ifdef FEAT_EVAL
+		--textwinlock;
+#endif
 		sub_nsubs++;
 		did_sub = TRUE;
 
diff --git a/src/testdir/test_substitute.vim b/src/testdir/test_substitute.vim
index f3fd7ab..a1c324e 100644
--- a/src/testdir/test_substitute.vim
+++ b/src/testdir/test_substitute.vim
@@ -1000,6 +1000,19 @@
   set nocompatible
 endfunc
 
+" This was switching windows in between computing the length and using it.
+func Test_sub_change_window()
+  silent! lfile
+  sil! norm o0000000000000000000000000000000000000000000000000000
+  func Repl()
+    lopen
+  endfunc
+  silent!  s/\%')/\=Repl()
+  bwipe!
+  bwipe!
+  delfunc Repl
+endfunc
+
 " Test for the 2-letter and 3-letter :substitute commands
 func Test_substitute_short_cmd()
   new
diff --git a/src/version.c b/src/version.c
index 4c63ea0..782642b 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4977,
+/**/
     4976,
 /**/
     4975,