diff --git a/include/nlohmann/detail/output/serializer.hpp b/include/nlohmann/detail/output/serializer.hpp
index a7dab5cd6..02bfaa548 100644
--- a/include/nlohmann/detail/output/serializer.hpp
+++ b/include/nlohmann/detail/output/serializer.hpp
@@ -927,7 +927,7 @@ class serializer
                 : (0xFFu >> type) & (byte);
 
         const std::size_t index = 256u + static_cast<size_t>(state) * 16u + static_cast<size_t>(type);
-        JSON_ASSERT(index < 400);
+        JSON_ASSERT(index < utf8d.size());
         state = utf8d[index];
         return state;
     }
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index e11f52968..d419f44fe 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -18849,7 +18849,7 @@ class serializer
                 : (0xFFu >> type) & (byte);
 
         const std::size_t index = 256u + static_cast<size_t>(state) * 16u + static_cast<size_t>(type);
-        JSON_ASSERT(index < 400);
+        JSON_ASSERT(index < utf8d.size());
         state = utf8d[index];
         return state;
     }