change syntax more clearly

Author: HH

doc/source/whatsnew/v0.25.1.rst

@@ -85,7 +85,7 @@ Indexing
 - Bug in partial-string indexing returning a NumPy array rather than a ``Series`` when indexing with a scalar like ``.loc['2015']`` (:issue:`27516`)
 - Break reference cycle involving :class:`Index` and other index classes to allow garbage collection of index objects without running the GC. (:issue:`27585`, :issue:`27840`)
 - Fix regression in assigning values to a single column of a DataFrame with a ``MultiIndex`` columns (:issue:`27841`).
--
+- ``IndexError`` would not raise if missing data in index (:issue:`20519`).
 
 Missing
 ^^^^^^^

pandas/core/indexes/multi.py

@@ -1279,7 +1279,7 @@ def _get_grouper_for_level(self, mapper, level):
             # Remove unobserved levels from level_index
             level_index = level_index.take(uniques)
 
-        if not level_index.tolist():
+        if not len(level_index):
             grouper = level_index
         else:
             grouper = level_index.take(codes)