diff --git a/src/unicode.hh b/src/unicode.hh
index 7d4c34734e..c40bb93a43 100644
--- a/src/unicode.hh
+++ b/src/unicode.hh
@@ -74,6 +74,8 @@ enum WordType { Word, WORD };
 template<WordType word_type = Word>
 inline bool is_word(Codepoint c, ConstArrayView<Codepoint> extra_word_chars = {'_'}) noexcept
 {
+    if (c == 0xFFFFFFFF)
+        return false;
     if (c < 128 ? is_basic_alpha(c) or is_basic_digit(c) : iswalnum((wchar_t)c))
         return true;
     for (auto cp : extra_word_chars)