diff --git a/include/nlohmann/detail/output/binary_writer.hpp b/include/nlohmann/detail/output/binary_writer.hpp
index 6c2ad70bc..8fb4c332c 100644
--- a/include/nlohmann/detail/output/binary_writer.hpp
+++ b/include/nlohmann/detail/output/binary_writer.hpp
@@ -881,7 +881,7 @@ class binary_writer
                     for (size_t i = 0; i < j.m_data.m_value.binary->size(); ++i)
                     {
                         oa->write_character(to_char_type(bjdata_draft3 ? 'B' : 'U'));
-                        oa->write_character(j.m_data.m_value.binary->data()[i]);
+                        oa->write_character(to_char_type(j.m_data.m_value.binary->data()[i]));
                     }
                 }
 
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 6f0132d20..f467c3998 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -17076,7 +17076,7 @@ class binary_writer
                     for (size_t i = 0; i < j.m_data.m_value.binary->size(); ++i)
                     {
                         oa->write_character(to_char_type(bjdata_draft3 ? 'B' : 'U'));
-                        oa->write_character(j.m_data.m_value.binary->data()[i]);
+                        oa->write_character(to_char_type(j.m_data.m_value.binary->data()[i]));
                     }
                 }