diff --git a/src/eval.c b/src/eval.c
index d76ec94..87b7644 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3515,10 +3515,11 @@
 	{
 	    ++p;
 	    // A "\<x>" form occupies at least 4 characters, and produces up
-	    // to 9 characters (6 for the char and 3 for a modifier): reserve
-	    // space for 5 extra.
+	    // to 21 characters (3 * 6 for the char and 3 for a modifier):
+	    // reserve space for 18 extra.
+	    // Each byte in the char could be encoded as K_SPECIAL K_EXTRA x.
 	    if (*p == '<')
-		extra += 5;
+		extra += 18;
 	}
     }
 
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 712aec4..d342e51 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -2280,6 +2280,7 @@
   call assert_equal('a', nr2char(97, 0))
 
   call assert_equal("\x80\xfc\b\xf4\x80\xfeX\x80\xfeX\x80\xfeX", eval('"\<M-' .. nr2char(0x100000) .. '>"'))
+  call assert_equal("\x80\xfc\b\xfd\x80\xfeX\x80\xfeX\x80\xfeX\x80\xfeX\x80\xfeX", eval('"\<M-' .. nr2char(0x40000000) .. '>"'))
 endfunc
 
 " Test for screenattr(), screenchar() and screenchars() functions
diff --git a/src/version.c b/src/version.c
index c4648f7..ce5f627 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    824,
+/**/
     823,
 /**/
     822,