diff --git a/lib.nix b/lib.nix
index 989b3e5..9a52f50 100644
--- a/lib.nix
+++ b/lib.nix
@@ -10,6 +10,7 @@ let
       dataDir ? ".data",
       setDataDir ? dataDir != null,
       createDataDir ? setDataDir,
+      gitPackage ? pkgs.git,
       ...
     }:
 
@@ -19,6 +20,7 @@ let
     ) "${name}: `dataDir` must be a string or null";
     assert lib.assertMsg (builtins.isBool setDataDir) "${name}: `setDataDir` must be a boolean";
     assert lib.assertMsg (builtins.isBool createDataDir) "${name}: `createDataDir` must be a boolean";
+    assert lib.assertMsg (lib.isDerivation gitPackage) "${name}: `gitPackage` must be a derivation";
 
     assert lib.assertMsg (
       dataDir != null -> (!lib.hasPrefix "/" dataDir)
@@ -41,7 +43,7 @@ let
       // {
         shellHook = ''
           # start -- tinyshell hook (${name})
-          export PRJ_ROOT="$(${lib.getExe pkgs.git} rev-parse --show-toplevel 2>/dev/null || printf '%s\n' "$PWD")"
+          export PRJ_ROOT="$(${lib.getExe gitPackage} rev-parse --show-toplevel 2>/dev/null || printf '%s\n' "$PWD")"
           ${lib.optionalString setDataDir ''export PRJ_DATA_DIR="$PRJ_ROOT/${dataDir}"''}
           ${lib.optionalString createDataDir ''mkdir -vp "$PRJ_DATA_DIR"''}
           # end -- tinyshell hook (${name})