diff --git a/pandas/core/indexing.py b/pandas/core/indexing.py
index 623a48acdd48b..15a4214a6f228 100755
--- a/pandas/core/indexing.py
+++ b/pandas/core/indexing.py
@@ -2444,6 +2444,9 @@ def length_of_indexer(indexer, target=None):
             step = -step
         return (stop - start + step - 1) // step
     elif isinstance(indexer, (ABCSeries, Index, np.ndarray, list)):
+        if hasattr('dtype'):
+            if indexer.dtype == bool:
+                return len(indexer[indexer])
         return len(indexer)
     elif not is_list_like_indexer(indexer):
         return 1