diff --git a/Lib/cdscan.py b/Lib/cdscan.py
index 63be542a..9b857989 100644
--- a/Lib/cdscan.py
+++ b/Lib/cdscan.py
@@ -1133,7 +1133,7 @@ def gensuffix(m, mo=mo):
                 nbsec, cdtime.Seconds)  # fctau0 as type comptime
             fc_time_attrs = []
 
-        varnames = f.variables.keys()
+        varnames = list(f.variables.keys())
 
         # Try to force all axes to be included, but only small ones, length<100.
         # This section was motivated by a need to preserve the cloud axes isccp_prs,isccp_tau.