diff --git a/requirements/core.txt b/requirements/core.txt
index e39777edb7..0b49a3a81b 100644
--- a/requirements/core.txt
+++ b/requirements/core.txt
@@ -4,7 +4,7 @@
 # Without these, iris won't even import.
 
 cartopy
-#conda: proj4<5
+#conda: proj4<6
 cf-units>=2
 cftime
 dask[array]  #conda: dask