diff --git a/scripts/nix/check-windows-fs.nix b/scripts/nix/check-windows-fs.nix
index 818e203346..a65cae93a1 100644
--- a/scripts/nix/check-windows-fs.nix
+++ b/scripts/nix/check-windows-fs.nix
@@ -14,11 +14,14 @@ let
     strictDeps = true;
     buildInputs = [ cross.windows.mcfgthreads ];
     buildPhase = ''
-      set -euo pipefail
-      echo '3a2926cd85ed7379700fe62485998a7fe26a9ba09824c370aa5b74e3d5508e5e  ${source}/rts-fs/fs.c' | sha256sum -c -
-      $CC -std=c11 -Werror=vla -pedantic -Wall -Wextra -O1 -static \
-        -DFS_NAMESPACE=rts -I${source}/rts-fs \
-        ${./windows-fs/Probe.c} ${source}/rts-fs/fs.c -o probe.exe
+      # Keep strict options local: standard fixup hooks do not use nounset.
+      (
+        set -euo pipefail
+        echo '3a2926cd85ed7379700fe62485998a7fe26a9ba09824c370aa5b74e3d5508e5e  ${source}/rts-fs/fs.c' | sha256sum -c -
+        $CC -std=c11 -Werror=vla -pedantic -Wall -Wextra -O1 -static \
+          -DFS_NAMESPACE=rts -I${source}/rts-fs \
+          ${./windows-fs/Probe.c} ${source}/rts-fs/fs.c -o probe.exe
+      )
     '';
     installPhase = ''
       mkdir -p "$out/bin"
diff --git a/scripts/nix/test-windows-fs.py b/scripts/nix/test-windows-fs.py
index 1ecb5f3b67..b5c4413833 100644
--- a/scripts/nix/test-windows-fs.py
+++ b/scripts/nix/test-windows-fs.py
@@ -11,6 +11,34 @@ HERE = Path(__file__).resolve().parent
 
 
 class ProbeDefinition(unittest.TestCase):
+    def test_build_phase_does_not_leak_nounset_into_fixup(self):
+        text = (HERE / "check-windows-fs.nix").read_text()
+        self.assertEqual(text.count("buildPhase = ''\n"), 1)
+        phase = text.split("buildPhase = ''\n", 1)[1].split("\n    '';", 1)[0]
+        phase = phase.replace("${source}", "/locked-source").replace(
+            "${./windows-fs/Probe.c}", "/unused-probe.c")
+        with tempfile.TemporaryDirectory() as temp:
+            temp = Path(temp)
+            for name, body in [("sha256sum", "cat >/dev/null"),
+                               ("fake-cc", "touch probe.exe")]:
+                path = temp / name
+                path.write_text("#!/usr/bin/env bash\n" + body + "\n")
+                path.chmod(0o755)
+            # Standard fixup hooks read optional variables without nounset.
+            suffix = '\n[ -f probe.exe ]\n[ -z "$exit_code" ]\n'
+            env = dict(os.environ, CC=str(temp / "fake-cc"),
+                       PATH=str(temp) + os.pathsep + os.environ["PATH"])
+            passed = subprocess.run(["bash", "-c", "set -e\n" + phase + suffix],
+                                    cwd=temp, env=env, capture_output=True, text=True, timeout=10)
+            self.assertEqual(passed.returncode, 0, passed.stderr)
+            # The same phase without its option-isolating subshell reproduces
+            # the real packaging failure, independently of the C compiler.
+            unscoped = phase.replace("      (\n", "", 1).rsplit("      )", 1)[0]
+            failed = subprocess.run(["bash", "-c", "set -e\n" + unscoped + suffix],
+                                    cwd=temp, env=env, capture_output=True, text=True, timeout=10)
+            self.assertNotEqual(failed.returncode, 0)
+            self.assertIn("exit_code: unbound variable", failed.stderr)
+
     def render(self, *, linux=True, x64=True, native=True):
         # Use throwing compiler-project attributes: this C check must not
         # accidentally grow a GHC planning/bootstrap dependency.