One issue seems to be that [this code](https://searchfox.org/mozilla-central/source/devtools/server/actors/thread.js#902-908) assumes that there is always at least one argument in the frame but if there isn't (see the `frame.arguments` watch expression in the screenshot) then we evaluate the expression `[_event]` with bindings `{ _event: undefined }` and end up constructing a message with `arguments: [undefined]`.
Bug 1960694 Comment 1 Edit History
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One issue seems to be that [this code](https://searchfox.org/mozilla-central/source/devtools/server/actors/thread.js#902-908) assumes that there is always at least one argument in the frame but if there isn't (see the `frame.arguments` watch expression in the screenshot) then we evaluate the expression `[_event]` with bindings `{ _event: undefined }` and end up constructing a message with `arguments: [undefined]` (I would have expected `arguments: [{ type: "undefined" }]`).
One issue seems to be that [this code](https://searchfox.org/mozilla-central/source/devtools/server/actors/thread.js#902-908) assumes that there is always at least one argument in the frame but if there isn't (see the `frame.arguments` watch expression in the screenshot) then we evaluate the expression `[_event]` with bindings `{ _event: undefined }` and end up constructing a message with `arguments: [undefined]`.