diff --git a/main.cpp b/main.cpp
index f55983a..c995077 100644
--- a/main.cpp
+++ b/main.cpp
@@ -26,8 +26,10 @@ const map<char, string> morse_map = {
 bool should_exit = false;
 
 bool is_exit(const string &s) {
-  return s.length() == 4 && tolower(s[0]) == 'e' && tolower(s[1]) == 'x' &&
-         tolower(s[2]) == 'i' && tolower(s[3]) == 't';
+  return s.length() == 4 && tolower(static_cast<unsigned char>(s[0])) == 'e' &&
+         tolower(static_cast<unsigned char>(s[1])) == 'x' &&
+         tolower(static_cast<unsigned char>(s[2])) == 'i' &&
+         tolower(static_cast<unsigned char>(s[3])) == 't';
 }
 
 string read_input(const string &prompt) {
@@ -74,7 +76,7 @@ public:
   MorseEncoder() : char_to_morse(morse_map) {}
 
   string encode(char c) {
-    auto it = char_to_morse.find(toupper(c));
+    auto it = char_to_morse.find(toupper(static_cast<unsigned char>(c)));
     return it != char_to_morse.end() ? it->second : "";
   }