diff --git a/include/nlohmann/detail/output/serializer.hpp b/include/nlohmann/detail/output/serializer.hpp
index 03d6ba352..aeec34eeb 100644
--- a/include/nlohmann/detail/output/serializer.hpp
+++ b/include/nlohmann/detail/output/serializer.hpp
@@ -919,7 +919,7 @@ class serializer
             }
         };
 
-        JSON_ASSERT(byte < utf8d.size());
+        JSON_ASSERT(static_cast<std::size_t>(byte) < utf8d.size());
         const std::uint8_t type = utf8d[byte];
 
         codep = (state != UTF8_ACCEPT)
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 57a90cbd4..cd4924d96 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -19755,7 +19755,7 @@ class serializer
             }
         };
 
-        JSON_ASSERT(byte < utf8d.size());
+        JSON_ASSERT(static_cast<std::size_t>(byte) < utf8d.size());
         const std::uint8_t type = utf8d[byte];
 
         codep = (state != UTF8_ACCEPT)