diff --git a/util/format.lua b/util/format.lua
index e07f5f2f5..f95f05755 100644
--- a/util/format.lua
+++ b/util/format.lua
@@ -15,6 +15,7 @@ local dump = require"prosody.util.serialization".new({
 	fallback = function(v, why)
 		return "_[[" .. (why or tostring(v)) .. "]] ";
 	end;
+	freeze = true;
 	fatal = false;
 	maxdepth = 5;
 });