diff --git a/include/inja/parser.hpp b/include/inja/parser.hpp
index c490a22..6b5f9fb 100644
--- a/include/inja/parser.hpp
+++ b/include/inja/parser.hpp
@@ -126,7 +126,7 @@ class Parser {
     }
   }
 
-  std::string parse_filename(const Token& tok) const {
+  std::string parse_filename() const {
     if (tok.kind != Token::Kind::String) {
       throw_parser_error("expected string, got '" + tok.describe() + "'");
     }
@@ -516,7 +516,7 @@ class Parser {
     } else if (tok.text == static_cast<decltype(tok.text)>("include")) {
       get_next_token();
 
-      std::string template_name = parse_filename(tok);
+      std::string template_name = parse_filename();
       add_to_template_storage(path, template_name);
 
       current_block->nodes.emplace_back(std::make_shared<IncludeStatementNode>(template_name, tok.text.data() - tmpl.content.c_str()));
@@ -525,7 +525,7 @@ class Parser {
     } else if (tok.text == static_cast<decltype(tok.text)>("extends")) {
       get_next_token();
 
-      std::string template_name = parse_filename(tok);
+      std::string template_name = parse_filename();
       add_to_template_storage(path, template_name);
 
       current_block->nodes.emplace_back(std::make_shared<ExtendsStatementNode>(template_name, tok.text.data() - tmpl.content.c_str()));
diff --git a/include/inja/renderer.hpp b/include/inja/renderer.hpp
index bebdb3e..f988673 100644
--- a/include/inja/renderer.hpp
+++ b/include/inja/renderer.hpp
@@ -352,7 +352,7 @@ class Renderer : public NodeVisitor {
     } break;
     case Op::Lower: {
       auto result = get_arguments<1>(node)[0]->get<json::string_t>();
-      std::transform(result.begin(), result.end(), result.begin(), ::tolower);
+      std::transform(result.begin(), result.end(), result.begin(), [](char c) { return static_cast<char>(::tolower(c)); });
       make_result(std::move(result));
     } break;
     case Op::Max: {
@@ -391,7 +391,7 @@ class Renderer : public NodeVisitor {
     } break;
     case Op::Upper: {
       auto result = get_arguments<1>(node)[0]->get<json::string_t>();
-      std::transform(result.begin(), result.end(), result.begin(), ::toupper);
+      std::transform(result.begin(), result.end(), result.begin(), [](char c) { return static_cast<char>(::toupper(c)); });
       make_result(std::move(result));
     } break;
     case Op::IsBoolean: {
diff --git a/single_include/inja/inja.hpp b/single_include/inja/inja.hpp
index 133ac8a..970e1ca 100644
--- a/single_include/inja/inja.hpp
+++ b/single_include/inja/inja.hpp
@@ -1540,7 +1540,7 @@ class Parser {
     }
   }
 
-  std::string parse_filename(const Token& tok) const {
+  std::string parse_filename() const {
     if (tok.kind != Token::Kind::String) {
       throw_parser_error("expected string, got '" + tok.describe() + "'");
     }
@@ -1930,7 +1930,7 @@ class Parser {
     } else if (tok.text == static_cast<decltype(tok.text)>("include")) {
       get_next_token();
 
-      std::string template_name = parse_filename(tok);
+      std::string template_name = parse_filename();
       add_to_template_storage(path, template_name);
 
       current_block->nodes.emplace_back(std::make_shared<IncludeStatementNode>(template_name, tok.text.data() - tmpl.content.c_str()));
@@ -1939,7 +1939,7 @@ class Parser {
     } else if (tok.text == static_cast<decltype(tok.text)>("extends")) {
       get_next_token();
 
-      std::string template_name = parse_filename(tok);
+      std::string template_name = parse_filename();
       add_to_template_storage(path, template_name);
 
       current_block->nodes.emplace_back(std::make_shared<ExtendsStatementNode>(template_name, tok.text.data() - tmpl.content.c_str()));
@@ -2432,7 +2432,7 @@ class Renderer : public NodeVisitor {
     } break;
     case Op::Lower: {
       auto result = get_arguments<1>(node)[0]->get<json::string_t>();
-      std::transform(result.begin(), result.end(), result.begin(), ::tolower);
+      std::transform(result.begin(), result.end(), result.begin(), [](char c) { return static_cast<char>(::tolower(c)); });
       make_result(std::move(result));
     } break;
     case Op::Max: {
@@ -2471,7 +2471,7 @@ class Renderer : public NodeVisitor {
     } break;
     case Op::Upper: {
       auto result = get_arguments<1>(node)[0]->get<json::string_t>();
-      std::transform(result.begin(), result.end(), result.begin(), ::toupper);
+      std::transform(result.begin(), result.end(), result.begin(), [](char c) { return static_cast<char>(::toupper(c)); });
       make_result(std::move(result));
     } break;
     case Op::IsBoolean: {