diff --git a/src/textual/_two_way_mapping.py b/src/textual/_two_way_mapping.py
index e2592a45e..f3f04f369 100644
--- a/src/textual/_two_way_mapping.py
+++ b/src/textual/_two_way_mapping.py
@@ -14,7 +14,7 @@ class TwoWayMapping(Generic[Key, Value]):
 
     def __init__(self, initial: dict[Key, Value]) -> None:
         self._forward: dict[Key, Value] = initial
-        self._reverse: dict[Value, Key] = {value: key for key, value in initial}
+        self._reverse: dict[Value, Key] = {value: key for key, value in initial.items()}
 
     def __setitem__(self, key: Key, value: Value) -> None:
         self._forward.__setitem__(key, value)
diff --git a/tests/test_two_way_mapping.py b/tests/test_two_way_mapping.py
new file mode 100644
index 000000000..9a88bdeec
--- /dev/null
+++ b/tests/test_two_way_mapping.py
@@ -0,0 +1,56 @@
+import pytest
+
+from textual._two_way_mapping import TwoWayMapping
+
+
+@pytest.fixture
+def map():
+    return TwoWayMapping(
+        {
+            1: 10,
+            2: 20,
+            3: 30,
+        }
+    )
+
+
+def test_get(map):
+    assert map.get(1) == 10
+
+
+def test_get_default_none(map):
+    assert map.get(9999) is None
+
+
+def test_get_default_supplied(map):
+    assert map.get(9999, -123) == -123
+
+
+def test_get_key(map):
+    assert map.get_key(30) == 3
+
+
+def test_get_key_default_none(map):
+    assert map.get_key(9999) is None
+
+
+def test_get_key_default_supplied(map):
+    assert map.get_key(9999, -123) == -123
+
+
+def test_set_item(map):
+    map[40] = 400
+    assert map.get(40) == 400
+    assert map.get_key(400) == 40
+
+
+def test_len(map):
+    assert len(map) == 3
+
+
+def test_delitem(map):
+    assert map.get(3) == 30
+    assert map.get_key(30) == 3
+    del map[3]
+    assert map.get(3) is None
+    assert map.get_key(30) is None