diff --git a/shared/cplusplus/Lexer.cpp b/shared/cplusplus/Lexer.cpp
index 6b08e2ad082c0cc62017f8853c9749579ec4533e..af6f09f74d211d285249c8601829bd2af4cee89f 100644
--- a/shared/cplusplus/Lexer.cpp
+++ b/shared/cplusplus/Lexer.cpp
@@ -221,7 +221,7 @@ void Lexer::scan_helper(Token *tok)
         return;
     }
 
-    char ch = _yychar;
+    unsigned char ch = _yychar;
     yyinp();
 
     switch (ch) {
diff --git a/shared/cplusplus/Lexer.h b/shared/cplusplus/Lexer.h
index f2eebf2bacf543cc970876be9840726fef307012..1d85a58eb9fb41664b4b21de4f2c66e32b1e4f6b 100644
--- a/shared/cplusplus/Lexer.h
+++ b/shared/cplusplus/Lexer.h
@@ -132,7 +132,7 @@ private:
     const char *_currentChar;
     const char *_lastChar;
     const char *_tokenStart;
-    char _yychar;
+    unsigned char _yychar;
     int _state;
     union {
         unsigned _flags;