diff --git a/doc/developers/utilities.rst b/doc/developers/utilities.rst
index f1fa601790060130e5410eb9a918eca350c13ce8..7ff6c330da1290af2fcc49dca9d1e1ea5f191475 100644
--- a/doc/developers/utilities.rst
+++ b/doc/developers/utilities.rst
@@ -257,6 +257,11 @@ Helper Functions
   a 1-d numpy.ndarray.  Currently used in
   ``sklearn.datasets._svmlight_format.pyx``.
 
+- :func:`safe_mask`: Help function to convert a mask to the format expected
+  by the numpy array or scipy sparse matrix on which to use it (sparse
+  matrices support integer indices only while numpy arrays support both
+  boolean masks and integer indices).
+
 
 Warnings and Exceptions
 =======================