diff --git a/Doc/library/xml.etree.elementtree.rst b/Doc/library/xml.etree.elementtree.rst
--- a/Doc/library/xml.etree.elementtree.rst
+++ b/Doc/library/xml.etree.elementtree.rst
@@ -419,6 +419,12 @@
    :class:`XMLParser` parser is used.  Returns an :term:`iterator` providing
    ``(event, elem)`` pairs.
 
+   .. note::
+
+      If the fast :mod:`xml.etree.cElementTree` C extension module is in use,
+      :func:`iterparse` can only accept *parser* objects that use the default
+      :class:`TreeBuilder` class as a target.
+
    Note that while :func:`iterparse` builds the tree incrementally, it issues
    blocking reads on *source* (or the file it names).  As such, it's unsuitable
    for asynchronous applications where blocking reads can't be made.  For fully
@@ -882,6 +888,12 @@
    parser instance.  If not given, the standard :class:`XMLParser` parser is
    used.
 
+   .. note::
+
+      If the fast :mod:`xml.etree.cElementTree` C extension module is in use,
+      :class:`IncrementalParser` can only accept *parser* objects that use the
+      default :class:`TreeBuilder` class as a target.
+
    .. method:: data_received(data)
 
       Feed the given bytes data to the incremental parser.