diff --git a/src/utils/string.cpp b/src/utils/string.cpp
index 89cf17d..eb91606 100644
--- a/src/utils/string.cpp
+++ b/src/utils/string.cpp
@@ -32,17 +32,19 @@ std::vector<std::string> split(const std::string& s, const char delim) {
 }
 
 std::string to_lower(std::string text) {
-  std::transform(text.begin(), text.end(), text.begin(), [](const char input) {
-    return static_cast<char>(tolower(input));
-  });
+  std::transform(text.begin(), text.end(), text.begin(),
+                 [](unsigned char input) {
+                   return static_cast<char>(std::tolower(input));
+                 });
 
   return text;
 }
 
 std::string to_upper(std::string text) {
-  std::transform(text.begin(), text.end(), text.begin(), [](const char input) {
-    return static_cast<char>(toupper(input));
-  });
+  std::transform(text.begin(), text.end(), text.begin(),
+                 [](unsigned char input) {
+                   return static_cast<char>(std::toupper(input));
+                 });
 
   return text;
 }