diff --git a/automata/base/automaton.py b/automata/base/automaton.py
index 5929ce3e..2675fe6c 100644
--- a/automata/base/automaton.py
+++ b/automata/base/automaton.py
@@ -151,6 +151,10 @@ def __repr__(self) -> str:
         )
         return f"{self.__class__.__qualname__}({values})"
 
-    def __contains__(self, input_str: str) -> bool:
+    def __contains__(self, item: Any) -> bool:
         """Returns whether the word is accepted by the automaton."""
-        return self.accepts_input(input_str)
+
+        if not isinstance(item, str):
+            return False
+
+        return self.accepts_input(item)
diff --git a/tests/test_dfa.py b/tests/test_dfa.py
index 8677a165..976af89e 100644
--- a/tests/test_dfa.py
+++ b/tests/test_dfa.py
@@ -178,6 +178,7 @@ def test_accepts_input_false(self) -> None:
         """Should return False if DFA input is rejected."""
         self.assertFalse(self.dfa.accepts_input("011"))
         self.assertNotIn("011", self.dfa)
+        self.assertNotIn(1, self.nfa)
 
     def test_read_input_step(self) -> None:
         """Should return validation generator if step flag is supplied."""
diff --git a/tests/test_nfa.py b/tests/test_nfa.py
index d21bd24c..951c7230 100644
--- a/tests/test_nfa.py
+++ b/tests/test_nfa.py
@@ -233,6 +233,7 @@ def test_accepts_input_false(self) -> None:
         """Should return False if NFA input is rejected."""
         self.assertFalse(self.nfa.accepts_input("abba"))
         self.assertNotIn("abba", self.nfa)
+        self.assertNotIn(1, self.nfa)
 
     def test_cyclic_lambda_transitions(self) -> None:
         """Should traverse NFA containing cyclic lambda transitions."""