diff --git a/src/evalvars.c b/src/evalvars.c
index 4037558..dabd49b 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -2199,12 +2199,7 @@
     }
 
     // b: variables
-    ht =
-#ifdef FEAT_CMDWIN
-	// In cmdwin, the alternative buffer should be used.
-	is_in_cmdwin() ? &prevwin->w_buffer->b_vars->dv_hashtab :
-#endif
-	&curbuf->b_vars->dv_hashtab;
+    ht = &prevwin_curwin()->w_buffer->b_vars->dv_hashtab;
     if (bdone < ht->ht_used)
     {
 	if (bdone++ == 0)
@@ -2217,12 +2212,7 @@
     }
 
     // w: variables
-    ht =
-#ifdef FEAT_CMDWIN
-	// In cmdwin, the alternative window should be used.
-	is_in_cmdwin() ? &prevwin->w_vars->dv_hashtab :
-#endif
-	&curwin->w_vars->dv_hashtab;
+    ht = &prevwin_curwin()->w_vars->dv_hashtab;
     if (wdone < ht->ht_used)
     {
 	if (wdone++ == 0)
diff --git a/src/proto/window.pro b/src/proto/window.pro
index 654d36a..11c2d47 100644
--- a/src/proto/window.pro
+++ b/src/proto/window.pro
@@ -1,4 +1,5 @@
 /* window.c */
+win_T *prevwin_curwin(void);
 void do_window(int nchar, long Prenum, int xchar);
 void get_wincmd_addr_type(char_u *arg, exarg_T *eap);
 int win_split(int size, int flags);
diff --git a/src/usercmd.c b/src/usercmd.c
index d4a7a9c..1ff4d0d 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -141,11 +141,7 @@
     /*
      * Look for buffer-local user commands first, then global ones.
      */
-    gap =
-#ifdef FEAT_CMDWIN
-	is_in_cmdwin() ? &prevwin->w_buffer->b_ucmds :
-#endif
-	&curbuf->b_ucmds;
+    gap = &prevwin_curwin()->w_buffer->b_ucmds;
     for (;;)
     {
 	for (j = 0; j < gap->ga_len; ++j)
@@ -358,11 +354,7 @@
 get_user_commands(expand_T *xp UNUSED, int idx)
 {
     // In cmdwin, the alternative buffer should be used.
-    buf_T *buf =
-#ifdef FEAT_CMDWIN
-	is_in_cmdwin() ? prevwin->w_buffer :
-#endif
-	curbuf;
+    buf_T *buf = prevwin_curwin()->w_buffer;
 
     if (idx < buf->b_ucmds.ga_len)
 	return USER_CMD_GA(&buf->b_ucmds, idx)->uc_name;
@@ -386,11 +378,7 @@
     if (cmdidx == CMD_USER_BUF)
     {
 	// In cmdwin, the alternative buffer should be used.
-	buf_T *buf =
-#ifdef FEAT_CMDWIN
-		    is_in_cmdwin() ? prevwin->w_buffer :
-#endif
-		    curbuf;
+	buf_T *buf = prevwin_curwin()->w_buffer;
 
 	if (idx < buf->b_ucmds.ga_len)
 	    return USER_CMD_GA(&buf->b_ucmds, idx)->uc_name;
@@ -478,11 +466,7 @@
     garray_T	*gap;
 
     // In cmdwin, the alternative buffer should be used.
-    gap =
-#ifdef FEAT_CMDWIN
-	    is_in_cmdwin() ? &prevwin->w_buffer->b_ucmds :
-#endif
-	    &curbuf->b_ucmds;
+    gap = &prevwin_curwin()->w_buffer->b_ucmds;
     for (;;)
     {
 	for (i = 0; i < gap->ga_len; ++i)
diff --git a/src/version.c b/src/version.c
index c5f5c22..777476d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4428,
+/**/
     4427,
 /**/
     4426,
diff --git a/src/window.c b/src/window.c
index 1f5e709..b00ed97 100644
--- a/src/window.c
+++ b/src/window.c
@@ -111,6 +111,21 @@
 #endif
 
 /*
+ * Return the current window, unless in the cmdline window and "prevwin" is
+ * set, then return "prevwin".
+ */
+    win_T *
+prevwin_curwin(void)
+{
+    return
+#ifdef FEAT_CMDWIN
+	// In cmdwin, the alternative buffer should be used.
+	is_in_cmdwin() && prevwin != NULL ? prevwin :
+#endif
+	curwin;
+}
+
+/*
  * All CTRL-W window commands are handled here, called from normal_cmd().
  */
     void
@@ -3927,6 +3942,14 @@
     tabpage_T	*newtp;
     int		n;
 
+#ifdef FEAT_CMDWIN
+    if (cmdwin_type != 0)
+    {
+	emsg(_(e_invalid_in_cmdline_window));
+	return FAIL;
+    }
+#endif
+
     newtp = alloc_tabpage();
     if (newtp == NULL)
 	return FAIL;
@@ -4301,6 +4324,7 @@
 	text_locked_msg();
 	return;
     }
+    CHECK_CMDWIN;
 
     // If there is only one it can't work.
     if (first_tabpage->tp_next == NULL)
@@ -4368,6 +4392,8 @@
     int		trigger_enter_autocmds,
     int		trigger_leave_autocmds)
 {
+    CHECK_CMDWIN;
+
     // Don't repeat a message in another tab page.
     set_keep_msg(NULL, 0);