Issue #21374 has been updated by matz (Yukihiro Matsumoto). I agree with you. It should be consistent (and I prefer `can't modify frozen Array: [] (FrozenError)` message). But there might be some internal reason. We have to investigate first. Matz. ---------------------------------------- Bug #21374: FrozenError message is inconsistent when a singleton method is defined on a frozen object https://bugs.ruby-lang.org/issues/21374#change-113616 * Author: andrykonchin (Andrew Konchin) * Status: Open * ruby -v: 3.4.2 * Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- I would expect a message to contain an object class (e.g. `can't modify frozen <class>: <object#inspect>`) when an ordinary frozen object (non a module or class) is being modified. It works this way in the following example: ```ruby object = []; object.freeze; object.instance_variable_set(:@a, 42) # (irb):5:in `instance_variable_set': can't modify frozen Array: [] (FrozenError) ``` But it doesn't in the following example: ```ruby object = []; object.freeze; def object.x; end # (irb):4:in `<main>': can't modify frozen object: [] (FrozenError) ``` -- https://bugs.ruby-lang.org/