What does "error: input attribute 'lastModified' is not an integer" mean from "nix flake update"?

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1

While updating a system from 25.05 to 25.11:

# nix flake update --commit-lock-file
error:
   … while updating the lock file of flake '[redacted]'
   … while updating the flake input 'nixpkgs'
   … while fetching the input 'https://channels.nixos.org/nixos-25.11/nixexprs.tar.xz'
   error: input attribute 'lastModified' is not an integer

What does this mean and what should I do about it?

2

Without seeing the actual lock entry, that’s hard to say exactly, though on first glance, I’d say the entries last modified is not an integer.

3

It sure looks like it’s an integer in flake.lock:

{
  "nodes": {
    "nixpkgs": {
      "locked": {
        "lastModified": 1765311797,
        "narHash": "sha256-3bKCEc1JWOn8aj48H1yHKU6WDw0eSU2t7cKCL3FC+Lo=",
        "rev": "09eb77e94fa25202af8f3e81ddc7353d9970ac1b",
        "type": "tarball",
        "url": "https://releases.nixos.org/nixos/25.11/nixos-25.11.1335.09eb77e94fa2/nixexprs.tar.xz?lastModified=1765311797&rev=09eb77e94fa25202af8f3e81ddc7353d9970ac1b"
      },
      "original": {
        "type": "tarball",
        "url": "https://channels.nixos.org/nixos-25.11/nixexprs.tar.xz"
      }
    },
    // etc
  }
  // etc
}

I do not know how to find out what it is in the file it tried to download.

4

I think this must be some kind of incompatibility between nix in 25.05 and nixpkgs tarballs from the 25.11 branch; after updating nix itself to 25.11 by hand, ‘nix flake update’ worked fine.

I doubt I can be bothered to investigate further; in case anyone else wants to, the troublesome nixexprs.tar.xz has this URL:

https://releases.nixos.org/nixos/25.11/nixos-25.11.1571.28bb483c11a1/nixexprs.tar.xz?lastModified=1765608474&rev=28bb483c11a1214a73f9fd2d9928a6e2ea86ec71

and here’s the relevant chunk of the lockfile diff as generated by the newer nix:

@@ -39,11 +39,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1765311797,
-        "narHash": "sha256-3bKCEc1JWOn8aj48H1yHKU6WDw0eSU2t7cKCL3FC+Lo=",
-        "rev": "09eb77e94fa25202af8f3e81ddc7353d9970ac1b",
+        "lastModified": 1765608474,
+        "narHash": "sha256-DBrujM5egJzWx9+AP14FAsaMrnFpt7q++OV0BDHSMPc=",
+        "rev": "28bb483c11a1214a73f9fd2d9928a6e2ea86ec71",
         "type": "tarball",
-        "url": "https://releases.nixos.org/nixos/25.11/nixos-25.11.1335.09eb77e94fa2/nixexprs.tar.xz?lastModified=1765311797&rev=09eb77e94fa25202af8f3e81ddc7353d9970ac1b"
+        "url": "https://releases.nixos.org/nixos/25.11/nixos-25.11.1571.28bb483c11a1/nixexprs.tar.xz?lastModified=1765608474&rev=28bb483c11a1214a73f9fd2d9928a6e2ea86ec71"
       },
       "original": {
         "type": "tarball",
5

nix run github:nixos/nixpkgs/nixos-25.11#nix -- flake update might’ve been an easy way to escape whatever the bug was.

1 Like
6

Yeah, that would have been much easier than what I wound up doing :slight_smile: