diff --git a/src/spellfile.c b/src/spellfile.c
index f0d6d96..4a0de52 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -4367,6 +4367,23 @@
 }
 
 /*
+ * Return TRUE if "word" contains valid word characters.
+ * Control characters and trailing '/' are invalid.  Space is OK.
+ */
+    static int
+valid_spell_word(char_u *word)
+{
+    char_u *p;
+
+    if (enc_utf8 && !utf_valid_string(word, NULL))
+	return FALSE;
+    for (p = word; *p != NUL; p += mb_ptr2len(p))
+	if (*p < ' ' || (p[0] == '/' && p[1] == NUL))
+	    return FALSE;
+    return TRUE;
+}
+
+/*
  * Store a word in the tree(s).
  * Always store it in the case-folded tree.  For a keep-case word this is
  * useful when the word can also be used with all caps (no WF_FIXCAP flag) and
@@ -4391,7 +4408,7 @@
     char_u	*p;
 
     // Avoid adding illegal bytes to the word tree.
-    if (enc_utf8 && !utf_valid_string(word, NULL))
+    if (!valid_spell_word(word))
 	return FAIL;
 
     (void)spell_casefold(curwin, word, len, foldword, MAXWLEN);
@@ -6194,7 +6211,7 @@
     int		i;
     char_u	*spf;
 
-    if (enc_utf8 && !utf_valid_string(word, NULL))
+    if (!valid_spell_word(word))
     {
 	emsg(_(e_illegal_character_in_word));
 	return;