diff --git a/src/lib_json/json_tool.h b/src/lib_json/json_tool.h
index 4316178..51aa7a0 100644
--- a/src/lib_json/json_tool.h
+++ b/src/lib_json/json_tool.h
@@ -61,9 +61,6 @@ static inline JSONCPP_STRING codePointToUTF8(unsigned int cp) {
   return result;
 }
 
-/// Returns true if ch is a control character (in range [1,31]).
-static inline bool isControlCharacter(char ch) { return ch > 0 && ch <= 0x1F; }
-
 enum {
   /// Constant that specify the size of the buffer that must be passed to
   /// uintToString.