diff --git a/include/nlohmann/detail/input/lexer.hpp b/include/nlohmann/detail/input/lexer.hpp
index 4a36ab06..cf38a813 100644
--- a/include/nlohmann/detail/input/lexer.hpp
+++ b/include/nlohmann/detail/input/lexer.hpp
@@ -1367,7 +1367,7 @@ scan_number_done:
         std::string result;
         for (const auto c : token_string)
         {
-            if ('\x00' <= c and c <= '\x1F')
+            if (static_cast<unsigned char>(c) <= '\x1F')
             {
                 // escape control characters
                 std::array<char, 9> cs{{}};
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 70ce011a..38e51596 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -9456,7 +9456,7 @@ scan_number_done:
         std::string result;
         for (const auto c : token_string)
         {
-            if ('\x00' <= c and c <= '\x1F')
+            if (static_cast<unsigned char>(c) <= '\x1F')
             {
                 // escape control characters
                 std::array<char, 9> cs{{}};