diff --git a/common_test.go b/common_test.go
index 2cf7680..ef34e7f 100644
--- a/common_test.go
+++ b/common_test.go
@@ -45,6 +45,7 @@ func testEmpties(t *testing.T, f func(itb gcf.Iterable[int]) gcf.Iterable[int])
 				t.Errorf("%v", err)
 			}
 		}()
+		// make empty elements but not emptyIterable.
 		itb := gcf.FromSlice([]int{1})
 		itb = gcf.Filter(itb, func(v int) bool { return false })
 		itb = f(itb)