diff -r 3a683b2476e2 -r 006a71a83e6a spec/util_serialization_spec.lua
--- a/spec/util_serialization_spec.lua	Thu Oct 11 18:20:09 2018 +0100
+++ b/spec/util_serialization_spec.lua	Thu Oct 11 22:42:17 2018 +0200
@@ -7,9 +7,19 @@
 			assert.is_string(serialization.serialize(nil));
 			assert.is_string(serialization.serialize(1));
 			assert.is_string(serialization.serialize(true));
-			assert.is_string(serialization.serialize(function () end));
 		end);
 
+		it("rejects function by default", function ()
+			assert.has_error(function ()
+				serialization.serialize(function () end)
+			end);
+		end);
+
+		it("makes a string in debug mode", function ()
+			assert.is_string(serialization.serialize(function () end, "debug"));
+		end);
+
+
 		it("roundtrips", function ()
 			local function test(data)
 				local serialized = serialization.serialize(data);