diff --git a/lib/iris/analysis/__init__.py b/lib/iris/analysis/__init__.py
index 4b1c631d4a..9613550929 100644
--- a/lib/iris/analysis/__init__.py
+++ b/lib/iris/analysis/__init__.py
@@ -1490,7 +1490,7 @@ def interp_order(length):
 
 
 MEAN = WeightedAggregator('mean', ma.average,
-                          lazy_func=_build_dask_mdtol_function(da.mean))
+                          lazy_func=_build_dask_mdtol_function(da.ma.average))
 """
 An :class:`~iris.analysis.Aggregator` instance that calculates
 the mean over a :class:`~iris.cube.Cube`, as computed by
@@ -1530,7 +1530,7 @@ def interp_order(length):
 
 .. note::
 
-    Lazy operation is supported, via :func:`dask.array.nanmean`.
+    Lazy operation is supported, via :func:`dask.array.ma.average`.
 
 This aggregator handles masked data.
 
diff --git a/requirements/core.txt b/requirements/core.txt
index 0b49a3a81b..eb7d39ca5d 100644
--- a/requirements/core.txt
+++ b/requirements/core.txt
@@ -7,7 +7,7 @@ cartopy
 #conda: proj4<6
 cf-units>=2
 cftime
-dask[array]  #conda: dask
+dask[array]>=1.1.0  #conda: dask>=1.1.0
 matplotlib>=2,<3
 netcdf4
 numpy>=1.14