diff --git a/src/in_n_out/_store.py b/src/in_n_out/_store.py
index 9d049d5..7c03a1d 100644
--- a/src/in_n_out/_store.py
+++ b/src/in_n_out/_store.py
@@ -27,7 +27,7 @@
 )
 
 from ._type_resolution import _resolve_sig_or_inform, resolve_type_hints
-from ._util import _split_union, issubclassable
+from ._util import _split_union, is_optional, issubclassable
 
 logger = getLogger("in_n_out")
 
@@ -781,7 +781,7 @@ def _exec(*args: P.args, **kwargs: P.kwargs) -> R:
                 for param in sig.parameters.values():
                     if param.name not in bound.arguments:
                         provided = self.provide(param.annotation)
-                        if provided is not None:
+                        if is_optional(param.annotation) or provided is not None:
                             logger.debug(
                                 "  injecting %s: %s = %r",
                                 param.name,
diff --git a/src/in_n_out/_util.py b/src/in_n_out/_util.py
index d89dbc4..c3fad03 100644
--- a/src/in_n_out/_util.py
+++ b/src/in_n_out/_util.py
@@ -11,9 +11,15 @@
 
 
 def _is_union(type_: Any) -> bool:
+    """Return True if `type_` is a Union type."""
     return get_origin(type_) in UNION_TYPES
 
 
+def is_optional(type_: Any) -> bool:
+    """Return True if `type_` is Optional[T]."""
+    return _split_union(type_)[1]
+
+
 def _split_union(type_: Any) -> Tuple[List[Type], bool]:
     optional = False
     if _is_union(type_):
diff --git a/tests/test_injection.py b/tests/test_injection.py
index 6b999c1..7a55be7 100644
--- a/tests/test_injection.py
+++ b/tests/test_injection.py
@@ -139,7 +139,7 @@ def test_injection_errors(in_func, on_unresolved, on_unannotated):
         if on_unresolved == "raise":
             ctx = pytest.raises(NameError, match=UNRESOLVED_MSG)
         elif on_unresolved == "warn":
-            ctx = pytest.warns(UserWarning, match=UNRESOLVED_MSG)
+            ctx = pytest.warns(UserWarning)  # will warn both
             if "unannotated" in in_func.__name__:
                 if on_unannotated == "raise":
                     ctxb = pytest.raises(TypeError, match=UNANNOTATED_MSG)
@@ -271,3 +271,23 @@ def func2(bar: "Bar", foo: "Foo"):  # noqa
     with test_store.register(providers={Foo: lambda: foo}):
         assert injected(bar=2) == (foo, 2)  # type: ignore
         assert injected2(2) == (foo, 2)  # type: ignore
+
+
+def test_inject_into_required_optional() -> None:
+    class Thing:
+        ...
+
+    def f(i: Optional[Thing]) -> Optional[Thing]:
+        return i
+
+    with pytest.raises(TypeError, match="missing 1 required positional argument"):
+        f()  # type: ignore
+
+    assert inject(f)() is None  # no provider needed
+
+    with register(providers={Optional[Thing]: lambda: None}):
+        assert inject(f)() is None
+
+    thing = Thing()
+    with register(providers={Optional[Thing]: lambda: thing}):
+        assert inject(f)() is thing