diff --git a/lib/default.nix b/lib/default.nix
index 5abafe1b2acf..d2e9f0e8086c 100644
--- a/lib/default.nix
+++ b/lib/default.nix
@@ -101,7 +101,7 @@ let
     inherit (sources) pathType pathIsDirectory cleanSourceFilter
       cleanSource sourceByRegex sourceFilesBySuffices
       commitIdFromGitRepo cleanSourceWith pathHasContext
-      canCleanSource pathIsRegularFile;
+      canCleanSource pathIsRegularFile pathIsGitRepo;
     inherit (modules) evalModules unifyModuleSyntax
       applyIfFunction mergeModules
       mergeModules' mergeOptionDecls evalOptionValue mergeDefinitions
diff --git a/lib/sources.nix b/lib/sources.nix
index a5765c0fda5b..05519c3e392e 100644
--- a/lib/sources.nix
+++ b/lib/sources.nix
@@ -105,6 +105,7 @@ rec {
       in type == "directory" || lib.any (ext: lib.hasSuffix ext base) exts;
     in cleanSourceWith { inherit filter; src = path; };
 
+  pathIsGitRepo = path: (builtins.tryEval (commitIdFromGitRepo path)).success;
 
   # Get the commit id of a git repo
   # Example: commitIdFromGitRepo <nixpkgs/.git>
diff --git a/lib/trivial.nix b/lib/trivial.nix
index 940ec1a3d59e..a281cd70fb0e 100644
--- a/lib/trivial.nix
+++ b/lib/trivial.nix
@@ -191,7 +191,7 @@ rec {
     let
       revisionFile = "${toString ./..}/.git-revision";
       gitRepo      = "${toString ./..}/.git";
-    in if builtins.pathExists gitRepo
+    in if lib.pathIsGitRepo gitRepo
        then lib.commitIdFromGitRepo gitRepo
        else if lib.pathExists revisionFile then lib.fileContents revisionFile
        else default;
diff --git a/nixos/modules/misc/version.nix b/nixos/modules/misc/version.nix
index ddbd3963cc57..8a85035ceb7c 100644
--- a/nixos/modules/misc/version.nix
+++ b/nixos/modules/misc/version.nix
@@ -6,6 +6,7 @@ let
   cfg = config.system.nixos;
 
   gitRepo      = "${toString pkgs.path}/.git";
+  gitRepoValid = lib.pathIsGitRepo gitRepo;
   gitCommitId  = lib.substring 0 7 (commitIdFromGitRepo gitRepo);
 in
 
@@ -91,8 +92,8 @@ in
       # These defaults are set here rather than up there so that
       # changing them would not rebuild the manual
       version = mkDefault (cfg.release + cfg.versionSuffix);
-      revision      = mkIf (pathExists gitRepo) (mkDefault            gitCommitId);
-      versionSuffix = mkIf (pathExists gitRepo) (mkDefault (".git." + gitCommitId));
+      revision      = mkIf gitRepoValid (mkDefault            gitCommitId);
+      versionSuffix = mkIf gitRepoValid (mkDefault (".git." + gitCommitId));
     };
 
     # Generate /etc/os-release.  See