diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index a858728c4c..6f74a4a526 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -9465,6 +9465,12 @@ class binary_reader
                 return get_number<std::int32_t, true>(input_format_t::bson, value) && sax->number_integer(value);
             }
 
+            case 0x11: // uint64
+            {
+                std::uint64_t value{};
+                return get_number<std::uint64_t, true>(input_format_t::bson, value) && sax->number_integer(value);
+            }
+
             case 0x12: // int64
             {
                 std::int64_t value{};
@@ -16106,7 +16112,7 @@ class binary_writer
     {
         return (value <= static_cast<std::uint64_t>((std::numeric_limits<std::int32_t>::max)()))
                ? sizeof(std::int32_t)
-               : sizeof(std::int64_t);
+               : sizeof(std::uint64_t);
     }
 
     /*!
@@ -16120,14 +16126,14 @@ class binary_writer
             write_bson_entry_header(name, 0x10 /* int32 */);
             write_number<std::int32_t>(static_cast<std::int32_t>(j.m_data.m_value.number_unsigned), true);
         }
-        else if (j.m_data.m_value.number_unsigned <= static_cast<std::uint64_t>((std::numeric_limits<std::int64_t>::max)()))
+        else if (j.m_data.m_value.number_unsigned <= static_cast<std::uint64_t>((std::numeric_limits<std::uint64_t>::max)()))
         {
-            write_bson_entry_header(name, 0x12 /* int64 */);
-            write_number<std::int64_t>(static_cast<std::int64_t>(j.m_data.m_value.number_unsigned), true);
+            write_bson_entry_header(name, 0x11 /* uint64 */);
+            write_number<std::uint64_t>(static_cast<std::uint64_t>(j.m_data.m_value.number_unsigned), true);
         }
         else
         {
-            JSON_THROW(out_of_range::create(407, concat("integer number ", std::to_string(j.m_data.m_value.number_unsigned), " cannot be represented by BSON as it does not fit int64"), &j));
+            JSON_THROW(out_of_range::create(407, concat("unsigned integer number ", std::to_string(j.m_data.m_value.number_unsigned), " cannot be represented by BSON as it does not fit into uint64"), &j));
         }
     }