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)