Completion fails, "no AST available"

[ncfavier]

Requesting completions fails with the following errors:

I[01:11:13.054] 2100851: <-- textDocument/completion(25)
I[01:11:13.055] 2100851: --> call nixd/ipc/textDocument/completion(266)
I[01:11:13.055] 2100851: --> call nixd/ipc/textDocument/completion(267)
I[01:11:13.055] 2225720: <-- nixd/ipc/textDocument/completion(266)
I[01:11:13.055] 2100851: --> call nixd/ipc/textDocument/completion(268)
I[01:11:13.055] 2225720: --> reply:nixd/ipc/textDocument/completion(266) no AST available on requested file /home/n/git/config/modules/nix.nix, error: 3
I[01:11:13.055] 2225724: <-- nixd/ipc/textDocument/completion(267)
I[01:11:13.055] 2100851: --> call nixd/ipc/textDocument/completion(269)
I[01:11:13.055] 2225724: --> reply:nixd/ipc/textDocument/completion(267) no AST available on requested file /home/n/git/config/modules/nix.nix, error: 3
I[01:11:13.055] 2225728: <-- nixd/ipc/textDocument/completion(268)
I[01:11:13.055] 2100851: <-- reply(266)
I[01:11:13.055] 2225732: <-- nixd/ipc/textDocument/completion(269)
E[01:11:13.055] 2100851: worker 0 reported error: -32001: no AST available on requested file /home/n/git/config/modules/nix.nix
I[01:11:13.055] 2225728: --> reply:nixd/ipc/textDocument/completion(268) no AST available on requested file /home/n/git/config/modules/nix.nix, error: 3
I[01:11:13.056] 2100851: <-- reply(267)
[truncated, more of the same]

[ncfavier]

I should probably finish reading the user guide before reporting issues. I somehow expected this to work out of the box.