diff --git a/include/nlohmann/detail/input/binary_reader.hpp b/include/nlohmann/detail/input/binary_reader.hpp
index 1245ac1d2..469379bc9 100644
--- a/include/nlohmann/detail/input/binary_reader.hpp
+++ b/include/nlohmann/detail/input/binary_reader.hpp
@@ -1922,7 +1922,7 @@ class binary_reader
 
         bool is_optimized = get_ubjson_size_type(size_and_type);
 
-        if (size_and_type.first != string_t::npos)
+        if (is_optimized && size_and_type.first != string_t::npos)
         {
             if (size_and_type.second != 0)
             {
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 13c501f70..81fb82739 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -10298,7 +10298,7 @@ class binary_reader
 
         bool is_optimized = get_ubjson_size_type(size_and_type);
 
-        if (size_and_type.first != string_t::npos)
+        if (is_optimized && size_and_type.first != string_t::npos)
         {
             if (size_and_type.second != 0)
             {