diff --git a/requirements/core.txt b/requirements/core.txt
index 997c91c665..e9abec5732 100644
--- a/requirements/core.txt
+++ b/requirements/core.txt
@@ -4,6 +4,7 @@
 # Without these, iris won't even import.
 
 cartopy
+#conda: proj4<5
 cf_units>=2
 cftime
 dask[array]==0.18.1  #conda: dask==0.18.1