BUG: .reset_index() should raise with an invalid level name (GH20925)

doc/source/whatsnew/v0.23.1.txt

@@ -59,7 +59,7 @@ Conversion
 Indexing
 ^^^^^^^^
 
--
+- Bug in :meth:`Series.reset_index` where appropriate error was not raised with a non-named level (:issue:`20925`)
 -
 
 I/O

pandas/core/series.py

@@ -1195,12 +1195,13 @@ def reset_index(self, level=None, drop=False, name=None, inplace=False):
         inplace = validate_bool_kwarg(inplace, 'inplace')
         if drop:
             new_index = com._default_index(len(self))
-            if level is not None and isinstance(self.index, MultiIndex):
+            if level is not None:
                 if not isinstance(level, (tuple, list)):
                     level = [level]
                 level = [self.index._get_level_number(lev) for lev in level]
-                if len(level) < len(self.index.levels):
-                    new_index = self.index.droplevel(level)
+                if isinstance(self.index, MultiIndex):
+                    if len(level) < self.index.nlevels:
+                        new_index = self.index.droplevel(level)
 
             if inplace:
                 self.index = new_index

pandas/tests/series/test_alter_axes.py

@@ -188,6 +188,11 @@ def test_reset_index_level(self):
             with tm.assert_raises_regex(IndexError, 'Too many levels'):
                 s.reset_index(level=[0, 1, 2])
 
+        # Check that .reset_index([],drop=True) doesn't fail
+        result = pd.Series(range(4)).reset_index([], drop=True)
+        expected = pd.Series(range(4))
+        assert_series_equal(result, expected)
+
     def test_reset_index_range(self):
         # GH 12071
         s = pd.Series(range(2), name='A', dtype='int64')
@@ -275,3 +280,18 @@ def test_set_axis_prior_to_deprecation_signature(self):
             with tm.assert_produces_warning(FutureWarning):
                 result = s.set_axis(0, list('abcd'), inplace=False)
             tm.assert_series_equal(result, expected)
+
+    def test_reset_index_drop_errors(self):
+        #  GH 20925
+
+        # KeyError raised for series index when passed level name is missing
+        s = pd.Series(range(4))
+        with tm.assert_raises_regex(KeyError, 'must be same as name'):
+            s.reset_index('wrong', drop=True)
+        with tm.assert_raises_regex(KeyError, 'must be same as name'):
+            s.reset_index('wrong')
+
+        # KeyError raised for series when level to be dropped is missing
+        s = pd.Series(range(4), index=pd.MultiIndex.from_product([[1, 2]] * 2))
+        with tm.assert_raises_regex(KeyError, 'not found'):
+            s.reset_index('wrong', drop=True)