diff --git a/default.nix b/default.nix
index 4565454..ba880e0 100644
--- a/default.nix
+++ b/default.nix
@@ -79,6 +79,12 @@ in pkgs.python3Packages.buildPythonPackage rec {
     in elemAt (match pattern (readFile ./tockloader/_version.py)) 0;
   name = "${pname}-${version}";
 
+  pyproject = true;
+
+  nativeBuildInputs = with python3Packages; [
+    flit
+  ];
+
   propagatedBuildInputs = with python3Packages; [
     argcomplete
     colorama
@@ -98,8 +104,8 @@ in pkgs.python3Packages.buildPythonPackage rec {
   doCheck = withUnfreePkgs;
 
   # Make other dependencies explicitly available as passthru attributes
-  passthru = {
+  passthru = if withUnfreePkgs then {
     inherit nrf-command-line-tools;
     pynrfjprog = python3Packages.pynrfjprog;
-  };
+  } else { };
 }
diff --git a/pyproject.toml b/pyproject.toml
index 8fe704d..0a27d33 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -16,7 +16,6 @@ dependencies = [
     "crcmod >= 1.7",
     "ecdsa >= 0.19.1",
     "pycryptodome >= 3.15.0",
-    "pynrfjprog == 10.19.0",
     "pyserial >= 3.0.1",
     "siphash >= 0.0.1",
     "six >= 1.9.0",
@@ -25,6 +24,11 @@ dependencies = [
     "questionary >= 1.10.0",
 ]
 
+[project.optional-dependencies]
+nrfjprog = [
+    "pynrfjprog >= 10.19.0",
+]
+
 [project.urls]
 Home = "https://github.com/tock/tockloader"