diff --git a/admin.go b/admin.go
index aed63e6..669088b 100644
--- a/admin.go
+++ b/admin.go
@@ -880,17 +880,30 @@ func (b *Barnard) addManualBan(text string) {
 		b.AddOutputLine("Admin: ban address must be CIDR, for example 192.0.2.1/32")
 		return
 	}
-	minutes, err := strconv.Atoi(parts[1])
+	duration, err := manualBanDuration(parts[1])
 	if err != nil {
-		b.AddOutputLine("Admin: ban minutes must be a number")
+		b.AddOutputLine("Admin: " + err.Error())
 		return
 	}
 	reason := strings.Join(parts[2:], " ")
-	b.adminBanList.Add(ip, network.Mask, reason, time.Duration(minutes)*time.Minute)
+	b.adminBanList.Add(ip, network.Mask, reason, duration)
 	b.Client.Send(b.adminBanList)
 	b.AddOutputLine("Admin: manual ban sent")
 }
 
+// manualBanDuration validates user input before it reaches the unsigned
+// protocol duration field.
+func manualBanDuration(text string) (time.Duration, error) {
+	minutes, err := strconv.Atoi(text)
+	if err != nil {
+		return 0, fmt.Errorf("ban minutes must be a number")
+	}
+	if minutes < 0 {
+		return 0, fmt.Errorf("ban minutes must not be negative")
+	}
+	return time.Duration(minutes) * time.Minute, nil
+}
+
 func (b *Barnard) unbanIndex(index int) {
 	if index < 0 || index >= len(b.adminBanList) {
 		b.AddOutputLine("Admin: ban index out of range")
diff --git a/admin_test.go b/admin_test.go
index 44682fb..59d0c80 100644
--- a/admin_test.go
+++ b/admin_test.go
@@ -65,6 +65,18 @@ func TestPermissionList(t *testing.T) {
 	}
 }
 
+// Regression: negative manual-ban minutes were cast to an unsigned protocol
+// duration, turning a rejected short ban into an extremely long one.
+func TestManualBanDurationRejectsNegativeMinutes(t *testing.T) {
+	if _, err := manualBanDuration("-1"); err == nil {
+		t.Fatal("negative duration was accepted")
+	}
+	got, err := manualBanDuration("15")
+	if err != nil || got != 15*60*1000000000 {
+		t.Fatalf("got %v, %v", got, err)
+	}
+}
+
 func TestAdminEscapeInputs(t *testing.T) {
 	if !isAdminEscapeKey(uiterm.KeyEsc) {
 		t.Fatal("expected escape key to close admin menu")
diff --git a/fix.txt b/fix.txt
index 018675a..930318f 100644
--- a/fix.txt
+++ b/fix.txt
@@ -266,7 +266,7 @@ Priority 3: configuration, UI, and binding hardening
     Both helpers panic on exec failure. Return/log an error or remove unused
     helpers; a missing optional beep binary must not terminate Barnard.
 
-35. Admin manual-ban duration accepts negative values
+[x] 35. Admin manual-ban duration accepts negative values
     Files: admin.go, gumble/gumble/bans.go
     Negative minutes become a negative duration, then are cast to uint32
     seconds for the protocol, creating a huge ban duration. Reject negative