diff --git a/fdroidserver/common.py b/fdroidserver/common.py
index 59570b52..a6039dcb 100644
--- a/fdroidserver/common.py
+++ b/fdroidserver/common.py
@@ -116,6 +116,7 @@ env = None
 orig_path = None
 
 
+# All paths in the config must be strings, never pathlib.Path instances
 default_config = {
     'sdk_path': "$ANDROID_HOME",
     'ndk_paths': {},
diff --git a/fdroidserver/metadata.py b/fdroidserver/metadata.py
index e5f82a69..fa895482 100644
--- a/fdroidserver/metadata.py
+++ b/fdroidserver/metadata.py
@@ -325,12 +325,14 @@ class Build(dict):
                 return f
         return 'ant'
 
-    def ndk_path(self):
-        """Return the path to the first configured NDK or an empty string."""
+    def ndk_path(self) -> str:
+        """Return the path string of the first configured NDK or an empty string."""
         ndk = self.ndk
         if isinstance(ndk, list):
             ndk = self.ndk[0]
         path = common.config['ndk_paths'].get(ndk)
+        if path and not isinstance(path, str):
+            raise TypeError('NDK path is not string')
         if path:
             return path
         for vsn, path in common.config['ndk_paths'].items():
diff --git a/tests/common.TestCase b/tests/common.TestCase
index 01f8a0e7..758b4881 100755
--- a/tests/common.TestCase
+++ b/tests/common.TestCase
@@ -2371,6 +2371,37 @@ class CommonTest(unittest.TestCase):
         )
         for f in config['java_paths'].values():
             self.assertTrue(f in java_paths)
+            self.assertTrue(isinstance(f, str))  # paths in config must be str
+
+    @mock.patch.dict(os.environ, clear=True)
+    def test_sdk_path_in_config_must_be_strings(self):
+        """All paths in config must be strings, and never pathlib.Path instances"""
+        os.environ['PATH'] = '/usr/bin:/usr/sbin'
+        config = {'sdk_path': Path('/opt/android-sdk')}
+        fdroidserver.common.fill_config_defaults(config)
+        build = fdroidserver.metadata.Build()
+        with self.assertRaises(TypeError):
+            fdroidserver.common.set_FDroidPopen_env(build)
+
+    @mock.patch.dict(os.environ, clear=True)
+    def test_ndk_paths_in_config_must_be_strings(self):
+        """All paths in config must be strings, and never pathlib.Path instances"""
+        fdroidserver.common.config = {
+            'ndk_paths': {'r21d': Path('/opt/android-sdk/ndk/r21d')}
+        }
+        build = fdroidserver.metadata.Build()
+        build.ndk = 'r21d'
+        os.environ['PATH'] = '/usr/bin:/usr/sbin'
+        with self.assertRaises(TypeError):
+            fdroidserver.common.set_FDroidPopen_env(build)
+
+    @mock.patch.dict(os.environ, clear=True)
+    def test_FDroidPopen_envs_paths_can_be_pathlib(self):
+        os.environ['PATH'] = '/usr/bin:/usr/sbin'
+        envs = {'PATHLIB': Path('/pathlib/path'), 'STRING': '/string/path'}
+        p = fdroidserver.common.FDroidPopen(['/bin/sh', '-c', 'export'], envs=envs)
+        self.assertIn('/string/path', p.output)
+        self.assertIn('/pathlib/path', p.output)
 
     def test_vcs_git_latesttags(self):
         tags = [
diff --git a/tests/metadata.TestCase b/tests/metadata.TestCase
index 73650834..6b2a94b4 100755
--- a/tests/metadata.TestCase
+++ b/tests/metadata.TestCase
@@ -1141,6 +1141,15 @@ class MetadataTest(unittest.TestCase):
             build.ndk = ['r22b', 'r10e']
             self.assertEqual(r22b, build.ndk_path())
 
+    def test_build_ndk_path_only_accepts_str(self):
+        """Paths in the config must be strings, never pathlib.Path instances"""
+        config = {'ndk_paths': {'r24': Path('r24')}}
+        fdroidserver.common.config = config
+        build = fdroidserver.metadata.Build()
+        build.ndk = 'r24'
+        with self.assertRaises(TypeError):
+            build.ndk_path()
+
 
 if __name__ == "__main__":
     parser = optparse.OptionParser()