diff --git a/packages/doc/src/fields.ts b/packages/doc/src/fields.ts
index 25f0ad4..ebe1cbf 100644
--- a/packages/doc/src/fields.ts
+++ b/packages/doc/src/fields.ts
@@ -18,7 +18,7 @@
 // " 10.0.0.1"; a phone's keyboard puts a space after a word). A name and an
 // SSID are taken as typed, spaces included, as Java takes them.
 
-import { DIRECTLY_CONNECTED_SOURCE_MARKER, IP_ADDRESS_PATTERN, javaIpCheck, parseIPv4 } from '@mdg-filius/core';
+import { DIRECTLY_CONNECTED_SOURCE_MARKER, IP_ADDRESS_PATTERN, javaIpCheck, modemCodeOf, parseIPv4 } from '@mdg-filius/core';
 
 export type FieldRule =
   | 'name'
@@ -34,7 +34,9 @@ export type FieldRule =
   /** A firewall rule's mask: empty, or ones then zeros (EingabenUeberpruefung.isValidSubnetmask). */
   | 'firewallMask'
   /** A firewall rule's port: empty for all ports, or musterPort. */
-  | 'portOrEmpty';
+  | 'portOrEmpty'
+  /** The code of another project's modem (modemCodeOf: six characters, spaces, dashes and small letters allowed). */
+  | 'modemCode';
 
 /** What a field's text comes to: the value the device takes, or that it takes none. */
 export type CheckedField = { readonly value: string } | { readonly invalid: true };
@@ -97,6 +99,10 @@ export function checkField(rule: FieldRule, text: string): CheckedField {
     }
     case 'portOrEmpty':
       return text.trim() === '' ? { value: '' } : checkField('port', text);
+    case 'modemCode': {
+      const code = modemCodeOf(text);
+      return code === undefined ? INVALID : { value: code };
+    }
     case 'macAddress':
       // Upper case, as the model keeps MAC addresses (Java keeps them as typed and compares ignoring case).
       return MAC_ADDRESS_PATTERN.test(text.trim()) ? { value: text.trim().toUpperCase() } : INVALID;
diff --git a/packages/doc/src/settings.ts b/packages/doc/src/settings.ts
index 3da1c0d..90b9734 100644
--- a/packages/doc/src/settings.ts
+++ b/packages/doc/src/settings.ts
@@ -41,7 +41,6 @@ import {
   firewallConfiguration,
   firewallRule,
   forwardsIPv4ByDefault,
-  modemCodeOf,
   type FirewallAction,
   type FirewallProtocol,
   type DhcpStaticAssignment,
@@ -251,11 +250,7 @@ export function setModemConnection(doc: Y.Doc, id: Id, connection: ModemConnecti
 
 /** The code of the modem a joining modem connects to, as typed (spaces, dashes, small letters allowed); refused when it is no code. */
 export function setModemCode(doc: Y.Doc, id: Id, text: string, origin: unknown): CheckedField {
-  const node = nodes(doc).get(id);
-  const code = modemCodeOf(text);
-  if (node === undefined || code === undefined) return { invalid: true };
-  doc.transact(() => modemSetting(node).set('relay', toY({ role: 'join', code })), origin);
-  return { value: code };
+  return setChecked(doc, id, checkField('modemCode', text), (node, code) => modemSetting(node).set('relay', toY({ role: 'join', code })), origin);
 }
 
 const portsOf = (node: Y.Map<unknown>) => node.get('ports') as Y.Array<Y.Map<unknown>>;