diff --git a/compliance_checker/cf/cf_1_6.py b/compliance_checker/cf/cf_1_6.py
index c1280d96..49178abf 100644
--- a/compliance_checker/cf/cf_1_6.py
+++ b/compliance_checker/cf/cf_1_6.py
@@ -116,17 +116,13 @@ def check_child_attr_data_types(self, ds):
         """
 
         ctx = TestCtx(BaseCheck.MEDIUM, self.section_titles["2.5"])
-        special_attrs = {
-            "actual_range",
-            "valid_min",
-            "valid_max",
-            "valid_range",
-            "_FillValue",
-        }
+        special_attrs = ("actual_range", "valid_min", "valid_max",
+                         "valid_range", "_FillValue")
 
         for _var_name, var in ds.variables.items():
-            for att_name in special_attrs.intersection(var.ncattrs()):
-                self._parent_var_attr_type_check(att_name, var, ctx)
+            for att_name in special_attrs:
+                if att_name in var.ncattrs():
+                    self._parent_var_attr_type_check(att_name, var, ctx)
         return ctx.to_result()
 
     # TODO: consider renaming to avoid confusion with non-underscore