diff --git a/include/nlohmann/detail/output/serializer.hpp b/include/nlohmann/detail/output/serializer.hpp
index f59e8ad31..0a34c8011 100644
--- a/include/nlohmann/detail/output/serializer.hpp
+++ b/include/nlohmann/detail/output/serializer.hpp
@@ -881,6 +881,7 @@ class serializer
             }
         };
 
+        JSON_ASSERT(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 4caae0560..e8ba43c95 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -16392,6 +16392,7 @@ class serializer
             }
         };
 
+        JSON_ASSERT(byte < utf8d.size());
         const std::uint8_t type = utf8d[byte];
 
         codep = (state != UTF8_ACCEPT)