diff --git a/pandas-stubs/core/arrays/string_.pyi b/pandas-stubs/core/arrays/string_.pyi
index 4ae56b77c..cdde9e028 100644
--- a/pandas-stubs/core/arrays/string_.pyi
+++ b/pandas-stubs/core/arrays/string_.pyi
@@ -7,7 +7,7 @@ from pandas._libs.missing import NAType
 from pandas.core.dtypes.base import ExtensionDtype
 
 class StringDtype(ExtensionDtype):
-    def __init__(self, storage: Literal["python", "pyarrow"] | None) -> None: ...
+    def __init__(self, storage: Literal["python", "pyarrow"] | None = None) -> None: ...
     @property
     def na_value(self) -> NAType: ...
 
diff --git a/tests/test_dtypes.py b/tests/test_dtypes.py
index 6792d2ba0..7fc8da847 100644
--- a/tests/test_dtypes.py
+++ b/tests/test_dtypes.py
@@ -126,6 +126,7 @@ def test_sparse_dtype() -> None:
 
 def test_string_dtype() -> None:
     s_dt = pd.StringDtype("pyarrow")
+    check(assert_type(pd.StringDtype(), pd.StringDtype), pd.StringDtype)
     check(assert_type(pd.StringDtype("pyarrow"), pd.StringDtype), pd.StringDtype)
     check(assert_type(pd.StringDtype("python"), pd.StringDtype), pd.StringDtype)
     check(assert_type(s_dt.na_value, NAType), NAType)