diff --git a/src/errors.h b/src/errors.h
index 1e9be6c..087f497 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3273,3 +3273,7 @@
 EXTERN char e_missing_close_curly_str[]
 	INIT(= N_("E1279: Missing '}': %s"));
 #endif
+#ifdef FEAT_SPELL
+EXTERN char e_illegal_character_in_word[]
+	INIT(= N_("E1280: Illegal character in word"));
+#endif
diff --git a/src/mbyte.c b/src/mbyte.c
index 2b7f999..a01a051 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -4226,7 +4226,7 @@
     convert_setup(&vimconv, NULL, NULL);
 }
 
-#if defined(FEAT_GUI_GTK) || defined(PROTO)
+#if defined(FEAT_GUI_GTK) || defined(FEAT_SPELL) || defined(PROTO)
 /*
  * Return TRUE if string "s" is a valid utf-8 string.
  * When "end" is NULL stop at the first NUL.
diff --git a/src/spellfile.c b/src/spellfile.c
index 22cf82d..f0d6d96 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -4390,6 +4390,10 @@
     int		res = OK;
     char_u	*p;
 
+    // Avoid adding illegal bytes to the word tree.
+    if (enc_utf8 && !utf_valid_string(word, NULL))
+	return FAIL;
+
     (void)spell_casefold(curwin, word, len, foldword, MAXWLEN);
     for (p = pfxlist; res == OK; ++p)
     {
@@ -6190,6 +6194,12 @@
     int		i;
     char_u	*spf;
 
+    if (enc_utf8 && !utf_valid_string(word, NULL))
+    {
+	emsg(_(e_illegal_character_in_word));
+	return;
+    }
+
     if (idx == 0)	    // use internal wordlist
     {
 	if (int_wordlist == NULL)
diff --git a/src/testdir/test_spell_utf8.vim b/src/testdir/test_spell_utf8.vim
index 79dc3e4..17fa235 100644
--- a/src/testdir/test_spell_utf8.vim
+++ b/src/testdir/test_spell_utf8.vim
@@ -780,5 +780,10 @@
   bwipe!
 endfunc
 
+" Invalid bytes may cause trouble when creating the word list.
+func Test_check_for_valid_word()
+  call assert_fails("spellgood! 0\xac", 'E1280:')
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/version.c b/src/version.c
index f949dd6..c4f5655 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4919,
+/**/
     4918,
 /**/
     4917,