diff --git a/src/simple_tokenizer.cc b/src/simple_tokenizer.cc
index 955fe5e..facf45c 100644
--- a/src/simple_tokenizer.cc
+++ b/src/simple_tokenizer.cc
@@ -21,7 +21,7 @@ PinYin *SimpleTokenizer::get_pinyin() {
   return py;
 }
 
-static TokenCategory from_char(char c) {
+static TokenCategory from_char(unsigned char c) {
   if (std::isdigit(c)) {
     return TokenCategory::DIGIT;
   }