diff --git a/envier/env.py b/envier/env.py
index e9ee301..9e9100a 100644
--- a/envier/env.py
+++ b/envier/env.py
@@ -67,6 +67,35 @@ def __init__(
         self.help_type = help_type
         self.help_default = help_default
 
+    def _cast(self, _type: t.Any, raw: str, env: "Env") -> t.Any:
+        if _type is bool:
+            return t.cast(T, raw.lower() in env.__truthy__)
+        elif _type in (list, tuple, set):
+            collection = raw.split(env.__item_separator__)
+            return t.cast(
+                T,
+                _type(  # type: ignore[operator]
+                    collection if self.map is None else map(self.map, collection)  # type: ignore[arg-type]
+                ),
+            )
+        elif _type is dict:
+            d = dict(
+                _.split(env.__value_separator__, 1)
+                for _ in raw.split(env.__item_separator__)
+            )
+            if self.map is not None:
+                d = dict(self.map(*_) for _ in d.items())
+            return t.cast(T, d)
+
+        if _check_type(raw, _type):
+            return t.cast(T, raw)
+
+        try:
+            return _type(raw)
+        except Exception as e:
+            msg = f"cannot cast {raw} to {self.type}"
+            raise TypeError(msg) from e
+
     def _retrieve(self, env: "Env", prefix: str) -> T:
         source = env.source
 
@@ -121,36 +150,14 @@ def _retrieve(self, env: "Env", prefix: str) -> T:
                 )
             return parsed
 
-        if self.type is bool:
-            return t.cast(T, raw.lower() in env.__truthy__)
-        elif self.type in (list, tuple, set):
-            collection = raw.split(env.__item_separator__)
-            return t.cast(
-                T,
-                self.type(  # type: ignore[operator]
-                    collection if self.map is None else map(self.map, collection)  # type: ignore[arg-type]
-                ),
-            )
-        elif self.type is dict:
-            d = dict(
-                _.split(env.__value_separator__, 1)
-                for _ in raw.split(env.__item_separator__)
-            )
-            if self.map is not None:
-                d = dict(self.map(*_) for _ in d.items())
-            return t.cast(T, d)
-
-        if _check_type(raw, self.type):
-            return t.cast(T, raw)
-
         if hasattr(self.type, "__origin__") and self.type.__origin__ is t.Union:  # type: ignore[attr-defined,union-attr]
             for ot in self.type.__args__:  # type: ignore[attr-defined,union-attr]
                 try:
-                    return t.cast(T, ot(raw))
+                    return t.cast(T, self._cast(ot, raw, env))
                 except TypeError:
                     pass
 
-        return self.type(raw)  # type: ignore[call-arg,operator]
+        return self._cast(self.type, raw, env)
 
     def __call__(self, env: "Env", prefix: str) -> T:
         value = self._retrieve(env, prefix)
@@ -436,9 +443,11 @@ def add_entries(full_prefix: str, config: t.Type[Env]) -> None:
                     (
                         f"``{private_prefix}{full_prefix}{_normalized(v.name)}``",
                         help_type,  # type: ignore[attr-defined]
-                        v.help_default
-                        if v.help_default is not None
-                        else str(v.default),
+                        (
+                            v.help_default
+                            if v.help_default is not None
+                            else str(v.default)
+                        ),
                         help_message,
                     )
                 )
diff --git a/tests/test_env.py b/tests/test_env.py
index 4374a00..15ff70d 100644
--- a/tests/test_env.py
+++ b/tests/test_env.py
@@ -284,11 +284,16 @@ class DictConfig(Env):
     assert DictConfig().foo == expected
 
 
-def test_env_optional_default():
+def test_env_optional_default(monkeypatch):
     class DictConfig(Env):
         foo = Env.var(Optional[str], "foo", default=None)
+        bar = Env.var(Optional[bool], "bar", default=None)
 
     assert DictConfig().foo is None
+    assert DictConfig().bar is None
+
+    monkeypatch.setenv("BAR", "0")
+    assert not DictConfig().bar
 
 
 @pytest.mark.parametrize("value,_type", [(1, int), ("1", str)])