diff --git a/pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/models.py b/pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/models.py
index 61b28050e6cf..186b25503dc4 100644
--- a/pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/models.py
+++ b/pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/models.py
@@ -171,12 +171,10 @@ class FlakeMetadataJson(TypedDict):
 
 
 class NixOSVersionJson(TypedDict):
-    nixosVersion: str
-    # Those keys are only set in nixos-version when it exists
+    # Keys are NotRequired here so we need to parse them safely
+    nixosVersion: NotRequired[str]
     configurationRevision: NotRequired[str]
     kernelVersion: NotRequired[str]
-    # The reason this key are NotRequired even when they're always set in
-    # nixos-version is because older generations may not have them yet
     specialisations: NotRequired[list[str]]
 
 
diff --git a/pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/nix.py b/pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/nix.py
index 3616b7c9f1e7..d157839a38b7 100644
--- a/pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/nix.py
+++ b/pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/nix.py
@@ -511,7 +511,7 @@ def list_generations(profile: Profile) -> list[GenerationJson]:
             profile.path.parent / f"{profile.path.name}-{generation.id}-link"
         )
 
-        j: NixOSVersionJson
+        j: NixOSVersionJson = {}
         try:
             result = run_wrapper(
                 [generation_path / "sw/bin/nixos-version", "--json"],
@@ -520,12 +520,11 @@ def list_generations(profile: Profile) -> list[GenerationJson]:
             j = json.loads(result)
         except (OSError, CalledProcessError) as ex:
             logger.debug("could not get configuration revision: %s", ex)
-            j = {"nixosVersion": "Unknown"}
 
         return GenerationJson(
             generation=generation.id,
             date=generation.timestamp,
-            nixosVersion=j["nixosVersion"],
+            nixosVersion=j.get("nixosVersion", "Unknown"),
             kernelVersion=j.get("kernelVersion", "Unknown"),
             configurationRevision=j.get("configurationRevision", "Unknown"),
             specialisations=j.get("specialisations", []),