diff --git a/plumbum/commands/base.py b/plumbum/commands/base.py
index 5650ce3f..51e667df 100644
--- a/plumbum/commands/base.py
+++ b/plumbum/commands/base.py
@@ -566,7 +566,7 @@ def popen(self, args=(), **kwargs):
 
 class ConcreteCommand(BaseCommand):
     QUOTE_LEVEL: ClassVar[int]
-    __slots__ = ("executable", "custom_encoding")
+    __slots__ = ("executable",)
 
     def __init__(self, executable, encoding):
         self.executable = executable
diff --git a/plumbum/machines/paramiko_machine.py b/plumbum/machines/paramiko_machine.py
index 36f8185a..a973d3c2 100644
--- a/plumbum/machines/paramiko_machine.py
+++ b/plumbum/machines/paramiko_machine.py
@@ -287,7 +287,7 @@ def sftp(self):
         return self._sftp
 
     def session(
-        self, isatty=False, term="vt100", width=80, height=24, new_session=False
+        self, isatty=False, term="vt100", width=80, height=24, *, new_session=False
     ):
         # new_session is ignored for ParamikoMachine
         trans = self._client.get_transport()
diff --git a/plumbum/machines/remote.py b/plumbum/machines/remote.py
index e214850c..8e002634 100644
--- a/plumbum/machines/remote.py
+++ b/plumbum/machines/remote.py
@@ -100,7 +100,7 @@ def getdelta(self):
 
 
 class RemoteCommand(ConcreteCommand):
-    __slots__ = ["remote", "executable"]
+    __slots__ = ("remote",)
     QUOTE_LEVEL = 1
 
     def __init__(self, remote, executable, encoding="auto"):
@@ -276,7 +276,7 @@ def python(self):
             self._python = self["python3"]
         return self._python
 
-    def session(self, isatty=False, new_session=False):
+    def session(self, isatty=False, *, new_session=False):
         """Creates a new :class:`ShellSession <plumbum.session.ShellSession>` object; this invokes the user's
         shell on the remote machine and executes commands on it over stdin/stdout/stderr"""
         raise NotImplementedError()