diff --git a/doc/user-guide/io.rst b/doc/user-guide/io.rst
index 48751c5f299..23b6c4d1188 100644
--- a/doc/user-guide/io.rst
+++ b/doc/user-guide/io.rst
@@ -898,6 +898,7 @@ where the data should be written (in index space, not label space), e.g.,
 
 Concurrent writes with ``region`` are safe as long as they modify distinct
 chunks in the underlying Zarr arrays (or use an appropriate ``lock``).
+Note that incorrectly (or omitting) setting the ``chunksizes`` for the initial Zarr file results in a partial update of the chunks when writing to regions, causing missing values in the Zarr file.
 
 As a safety check to make it harder to inadvertently override existing values,
 if you set ``region`` then *all* variables included in a Dataset must have