Unmapped values are taken into account in the object mapping size.

src/cbor/serializable.cr

@@ -244,6 +244,10 @@ module CBOR
       raise ::CBOR::SerializationError.new("Unknown CBOR attribute: #{key}", self.class.to_s, nil)
     end
 
+    protected def get_cbor_unmapped
+      {} of String => ::CBOR::Type
+    end
+
     protected def on_to_cbor(cbor : ::CBOR::Encoder)
     end
 
@@ -283,6 +287,7 @@ module CBOR
           {% end %} # macro unless value[:emit_null]
         {% end %} # macro for properties
 
+        nb_properties_to_serialize += get_cbor_unmapped.size
 
 
         {% if properties.size > 0 %}
@@ -329,6 +334,10 @@ module CBOR
         end
       end
 
+      protected def get_cbor_unmapped
+        cbor_unmapped
+      end
+
       protected def on_to_cbor(cbor : ::CBOR::Encoder)
         cbor_unmapped.each do |key, value|
           cbor.write(key)