diff --git a/holoviews/core/operation.py b/holoviews/core/operation.py
index c82945874e..a499685ebb 100644
--- a/holoviews/core/operation.py
+++ b/holoviews/core/operation.py
@@ -7,7 +7,7 @@
 from .element import Element
 from .layout import Layout
 from .overlay import NdOverlay, Overlay
-from .spaces import Callable
+from .spaces import Callable, HoloMap
 from . import util