diff --git a/internal/cmd/calendar/calendar.go b/internal/cmd/calendar/calendar.go
index 671c76d..ee2f673 100644
--- a/internal/cmd/calendar/calendar.go
+++ b/internal/cmd/calendar/calendar.go
@@ -90,10 +90,13 @@ func newCmdSet() *cobra.Command {
 		Short: "Replace calendar section subscriptions",
 		Args:  cobra.MinimumNArgs(1),
 		RunE: func(cmd *cobra.Command, args []string) error {
+			if strings.TrimSpace(semesterID) == "" {
+				return fmt.Errorf("--semester-id is required for calendar set")
+			}
 			return runCalendarBatch(cmd, args, openapi.CalendarSubscriptionBatchRequestSchemaActionSet, semesterID)
 		},
 	}
-	cmd.Flags().StringVar(&semesterID, "semester-id", "", "Semester ID to narrow code matches")
+	cmd.Flags().StringVar(&semesterID, "semester-id", "", "Semester ID whose subscriptions will be replaced")
 	return cmd
 }
 
diff --git a/internal/cmd/calendar/calendar_test.go b/internal/cmd/calendar/calendar_test.go
new file mode 100644
index 0000000..9cb40a5
--- /dev/null
+++ b/internal/cmd/calendar/calendar_test.go
@@ -0,0 +1,29 @@
+package calendar
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestSetRequiresSemesterID(t *testing.T) {
+	for _, args := range [][]string{
+		{"999999"},
+		{"999999", "--semester-id", " "},
+	} {
+		t.Run(strings.Join(args, " "), func(t *testing.T) {
+			t.Setenv("LIFE_USTC_CONFIG_DIR", t.TempDir())
+			t.Setenv("LIFE_USTC_SERVER", "http://127.0.0.1:1")
+
+			cmd := newCmdSet()
+			cmd.SetArgs(args)
+
+			err := cmd.Execute()
+			if err == nil {
+				t.Fatal("expected calendar set without --semester-id to fail")
+			}
+			if !strings.Contains(err.Error(), "--semester-id is required") {
+				t.Fatalf("unexpected error: %v", err)
+			}
+		})
+	}
+}