diff --git a/simple_backup/simple_backup.py b/simple_backup/simple_backup.py
index a32490e..de99332 100755
--- a/simple_backup/simple_backup.py
+++ b/simple_backup/simple_backup.py
@@ -136,14 +136,16 @@ class Backup:
             Perform the backup
     """
 
-    def __init__(self, inputs: List[str], output: str, exclude: List[str], keep: int, options: str,
+    def __init__(self, inputs: Optional[List[str]], output: Optional[str], exclude: Optional[List[str]],
+                 keep: Optional[int], options: str,
                  ssh_host: Optional[str] = None, ssh_user: Optional[str] = None, ssh_keyfile: Optional[str] = None,
                  remote_sudo: bool = False, remove_before: bool = False, verbose: bool = False) -> None:
-        self.inputs = inputs
-        self.output = output
-        self.exclude = exclude
+        # Missing values are reported by check_params()
+        self.inputs = inputs if inputs is not None else []
+        self.output = output if output is not None else ''
+        self.exclude = exclude if exclude is not None else []
         self.options = options
-        self.keep = keep
+        self.keep = keep if keep is not None else -1
         self.ssh_host = ssh_host
         self.ssh_user = ssh_user
         self.ssh_keyfile = ssh_keyfile
@@ -156,20 +158,20 @@ class Backup:
         self._inputs_path = ''
         self._exclude_path = ''
         self._remote = False
-        self._ssh = None
+        self._ssh: Optional[paramiko.client.SSHClient] = None
         self._password_auth = False
         self._removed_count = 0
 
     def check_params(self, homedir: str = '') -> int:
         """Check if parameters for the backup are valid"""
 
-        if self.inputs is None or len(self.inputs) == 0:
+        if len(self.inputs) == 0:
             logger.info(
                 'No existing files or directories specified for backup. Nothing to do')
 
             return 1
 
-        if self.output is None:
+        if self.output == '':
             logger.critical(
                 'No output path specified. Use -o argument or specify output path in configuration file')
 
@@ -207,9 +209,6 @@ class Backup:
 
             self.output = os.path.abspath(self.output)
 
-        if self.keep is None:
-            self.keep = -1
-
         return 0
 
     def _resolve_remote_path(self, path: str) -> Optional[str]:
@@ -605,10 +604,9 @@ class Backup:
         fd, self._exclude_path = mkstemp(prefix='tmp_exclude', text=True)
 
         with os.fdopen(fd, 'w', encoding='utf-8') as fp:
-            if self.exclude is not None:
-                for e in self.exclude:
-                    fp.write(e)
-                    fp.write('\n')
+            for e in self.exclude:
+                fp.write(e)
+                fp.write('\n')
 
         if self.keep != -1 and self._remove_before:
             self.remove_old_backups()
@@ -1015,9 +1013,7 @@ def simple_backup() -> int:
     if args.numeric_ids or config_args['numeric_ids']:
         rsync_options.append('--numeric-ids')
 
-    rsync_options = ' '.join(rsync_options)
-
-    backup = Backup(inputs, output, exclude, keep, rsync_options, ssh_host, ssh_user, ssh_keyfile,
+    backup = Backup(inputs, output, exclude, keep, ' '.join(rsync_options), ssh_host, ssh_user, ssh_keyfile,
                     remote_sudo, remove_before=args.remove_before_backup, verbose=args.verbose)
 
     try:
diff --git a/tests/test_local.py b/tests/test_local.py
index a668993..b4e610e 100644
--- a/tests/test_local.py
+++ b/tests/test_local.py
@@ -79,14 +79,14 @@ class TestCheckParams:
         assert sb.Backup([], str(out), [], -1, OPTIONS).check_params() == 1
 
     def test_no_output(self) -> None:
-        assert sb.Backup(['/x'], None, [], -1, OPTIONS).check_params() == 2  # type: ignore[arg-type]
+        assert sb.Backup(['/x'], None, [], -1, OPTIONS).check_params() == 2
 
     def test_missing_output(self, tmp_path: Path) -> None:
         assert sb.Backup(['/x'], str(tmp_path / 'missing'), [], -1, OPTIONS).check_params() == 2
 
     def test_relative_output(self, out: Path, monkeypatch: pytest.MonkeyPatch) -> None:
         monkeypatch.chdir(out.parent)
-        backup = sb.Backup(['/x'], out.name, [], None, OPTIONS)  # type: ignore[arg-type]
+        backup = sb.Backup(['/x'], out.name, [], None, OPTIONS)
 
         assert backup.check_params() == 0
         assert backup.output == str(out)