fixes for pandas-dev#16155 and pandas-dev#16156 (need testing)

Author: ResidentMario

pandas/core/generic.py

@@ -3449,7 +3449,8 @@ def fillna(self, value=None, method=None, axis=None, inplace=False,
         # if a singular fill value is provided, validate it
         if value is not None:
             # fill values by column, not all at once, to respect dtypes
-            if not isinstance(value, dict) and isinstance(self, ABCDataFrame):
+            if not isinstance(value, (dict, ABCSeries)) and \
+                    isinstance(self, ABCDataFrame):
                 value = {col: value for col in self.columns}
             try:
                 missing.validate_fill_value(self, value)
@@ -3544,7 +3545,7 @@ def fillna(self, value=None, method=None, axis=None, inplace=False,
                     obj = result[k]
                     obj.fillna(v, limit=limit, inplace=True,
                                downcast=downcast, errors=errors)
-                return result
+                return None if inplace else result
             elif not is_list_like(value):
                 new_data = self._data.fillna(value=value, limit=limit,
                                              inplace=inplace,