diff --git a/pandas/core/series.py b/pandas/core/series.py
index 6586ac8b01840..897e7c65466ae 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -3980,7 +3980,13 @@ def transform(
     ) -> FrameOrSeriesUnion:
         return transform(self, func, axis, *args, **kwargs)
 
-    def apply(self, func, convert_dtype=True, args=(), **kwds):
+    def apply(
+        self,
+        func: AggFuncType,
+        convert_dtype: bool = True,
+        args: Tuple[Any, ...] = (),
+        **kwds,
+    ) -> FrameOrSeriesUnion:
         """
         Invoke function on values of Series.
 
diff --git a/pandas/io/sas/sas7bdat.py b/pandas/io/sas/sas7bdat.py
index 7c2b801ee0ea8..fe06f103e6c5e 100644
--- a/pandas/io/sas/sas7bdat.py
+++ b/pandas/io/sas/sas7bdat.py
@@ -52,13 +52,17 @@ def _convert_datetimes(sas_datetimes: pd.Series, unit: str) -> pd.Series:
         return pd.to_datetime(sas_datetimes, unit=unit, origin="1960-01-01")
     except OutOfBoundsDatetime:
         if unit == "s":
-            return sas_datetimes.apply(
+            s_series = sas_datetimes.apply(
                 lambda sas_float: datetime(1960, 1, 1) + timedelta(seconds=sas_float)
             )
+            s_series = cast(pd.Series, s_series)
+            return s_series
         elif unit == "d":
-            return sas_datetimes.apply(
+            d_series = sas_datetimes.apply(
                 lambda sas_float: datetime(1960, 1, 1) + timedelta(days=sas_float)
             )
+            d_series = cast(pd.Series, d_series)
+            return d_series
         else:
             raise ValueError("unit must be 'd' or 's'")