diff --git a/c/driver_manager/adbc_driver_manager.cc b/c/driver_manager/adbc_driver_manager.cc
index 0ce173a..2a7fef8 100644
--- a/c/driver_manager/adbc_driver_manager.cc
+++ b/c/driver_manager/adbc_driver_manager.cc
@@ -675,7 +675,7 @@ std::string AdbcDriverManagerDefaultEntrypoint(const std::string& driver) {
     // if pos == npos this is the entire filename
     std::string token = filename.substr(prev, pos - prev);
     // capitalize first letter
-    token[0] = std::toupper(static_cast<unsigned char>(token[0]));
+    token[0] = static_cast<char>(std::toupper(static_cast<unsigned char>(token[0])));
 
     entrypoint += token;