diff --git a/include/dpp/stringops.h b/include/dpp/stringops.h
index cd99d9cc4f..31b86d1443 100644
--- a/include/dpp/stringops.h
+++ b/include/dpp/stringops.h
@@ -24,6 +24,7 @@
 
 #pragma once
 #include <string>
+#include <cctype>
 #include <iomanip>
 #include <locale>
 #include <algorithm>
@@ -42,7 +43,7 @@ namespace dpp {
 template <typename T> std::basic_string<T> lowercase(const std::basic_string<T>& s)
 {
 	std::basic_string<T> s2 = s;
-	std::transform(s2.begin(), s2.end(), s2.begin(), tolower);
+	std::transform(s2.begin(), s2.end(), s2.begin(), [](unsigned char c){ return std::tolower(c); });
 	return s2;
 }
 
@@ -56,7 +57,7 @@ template <typename T> std::basic_string<T> lowercase(const std::basic_string<T>&
 template <typename T> std::basic_string<T> uppercase(const std::basic_string<T>& s)
 {
 	std::basic_string<T> s2 = s;
-	std::transform(s2.begin(), s2.end(), s2.begin(), toupper);
+	std::transform(s2.begin(), s2.end(), s2.begin(), [](unsigned char c){ return std::toupper(c); });
 	return s2;
 }
 
diff --git a/src/dpp/cluster/confirmation.cpp b/src/dpp/cluster/confirmation.cpp
index 6e48b138a2..56e8904adf 100644
--- a/src/dpp/cluster/confirmation.cpp
+++ b/src/dpp/cluster/confirmation.cpp
@@ -93,7 +93,7 @@ std::vector<error_detail> find_errors_in_object(const std::string& obj, const st
 
 			if (obj.empty()) {
 				field = current_field;
-			} else if (isdigit(*current_field.c_str())) {
+			} else if (isdigit(static_cast<unsigned char>(*current_field.c_str()))) {
 				/* An element of an array, e.g. an element of a slash command vector for global_bulk_slash_command_create */
 				field = obj;
 				field += '[';
@@ -130,7 +130,7 @@ error_info confirmation_callback_t::get_error() const {
 			json &errors = j["errors"];
 			for (auto obj = errors.begin(); obj != errors.end(); ++obj) {
 				std::vector<error_detail> sub_errors;
-				std::string field = isdigit(*obj.key().c_str()) ? "<array>[" + obj.key() + "]" : obj.key();
+				std::string field = isdigit(static_cast<unsigned char>(*obj.key().c_str())) ? "<array>[" + obj.key() + "]" : obj.key();
 
 				sub_errors = find_errors_in_object({}, field, *obj);