diff --git a/pandas/core/series.py b/pandas/core/series.py
index bbcb6615aeefd..1be74df8c9ac3 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -3224,6 +3224,13 @@ def combine_first(self, other) -> Series:
         """
         from pandas.core.reshape.concat import concat
 
+        def replace_none_with_nan(series):
+            series.fillna(value=np.nan, inplace=True)
+
+        """Apply the function to both Series"""
+        replace_none_with_nan(self)
+        replace_none_with_nan(other)
+
         if self.dtype == other.dtype:
             if self.index.equals(other.index):
                 return self.mask(self.isna(), other)