diff --git a/docs/api.md b/docs/api.md
index 901f0e0..2c63b26 100755
--- a/docs/api.md
+++ b/docs/api.md
@@ -727,11 +727,11 @@ curl -H "Content-Type: application/json" \
 curl -H "Content-Type: application/json" \
   -H "auth-token: your-token-here" \
   -X POST \
-  -d '{"name": "My DuckDNS", "dnsProvider": "DuckDns", "token": "your-duckdns-token", "domains": "myhost,myhost2"}' \
+  -d '{"name": "My DuckDNS", "dnsProvider": "DuckDns", "token": "your-duckdns-token", "subdomains": "myhost,myhost2"}' \
   https://proxy-host.com/api/dns
 ```
 
-`domains` is a comma-separated list of the subdomains you've registered at
+`subdomains` is a comma-separated list of the subdomains you've registered at
 [duckdns.org](https://www.duckdns.org) (e.g. `myhost` for
 `myhost.duckdns.org`), since DuckDNS has no API to list them for you.
 DuckDNS only supports one A/AAAA record and one TXT record per domain (no
diff --git a/nodejs/api.md b/nodejs/api.md
index d6ef8dc..f9b66ad 100755
--- a/nodejs/api.md
+++ b/nodejs/api.md
@@ -720,11 +720,11 @@ curl -H "Content-Type: application/json" \
 curl -H "Content-Type: application/json" \
   -H "auth-token: your-token-here" \
   -X POST \
-  -d '{"name": "My DuckDNS", "dnsProvider": "DuckDns", "token": "your-duckdns-token", "domains": "myhost,myhost2"}' \
+  -d '{"name": "My DuckDNS", "dnsProvider": "DuckDns", "token": "your-duckdns-token", "subdomains": "myhost,myhost2"}' \
   https://proxy-host.com/api/dns
 ```
 
-`domains` is a comma-separated list of the subdomains you've registered at
+`subdomains` is a comma-separated list of the subdomains you've registered at
 [duckdns.org](https://www.duckdns.org) (e.g. `myhost` for
 `myhost.duckdns.org`), since DuckDNS has no API to list them for you.
 DuckDNS only supports one A/AAAA record and one TXT record per domain (no
diff --git a/nodejs/models/dns_provider.js b/nodejs/models/dns_provider.js
index 04bdcaa..01f98e3 100644
--- a/nodejs/models/dns_provider.js
+++ b/nodejs/models/dns_provider.js
@@ -93,6 +93,13 @@ class DnsProvider extends Table{
 		}
 
 		let Provider = providers[provider];
+		// Provider._keyMap is spread last, so a provider-defined field with the
+		// same name as one of ours (created_by, updated_by, name, dnsProvider,
+		// domains, id — see this._keyMap above) silently overwrites it. This bit
+		// a DuckDNS field named `domains`: it replaced the `domains` relation
+		// (rel: 'many' to Domain), so `this.domains` stopped being an array and
+		// `updateDomains()` broke with "this.domains.map is not a function".
+		// New providers must avoid these names.
 		let _keyMap = {...this._keyMap, ...Provider._keyMap};
 
 		return ({
diff --git a/nodejs/models/dns_provider/duckdns.js b/nodejs/models/dns_provider/duckdns.js
index 24caaed..f93d8b0 100644
--- a/nodejs/models/dns_provider/duckdns.js
+++ b/nodejs/models/dns_provider/duckdns.js
@@ -13,7 +13,7 @@ account-wide token. Its API is much smaller than a full DNS provider's:
 - There is no read or list API. `getRecords` here resolves the domain via
   public DNS instead, since that's the only source of truth available.
 - There's no API to enumerate which subdomains a token owns either, so the
-  operator supplies them directly (the `domains` field below) rather than
+  operator supplies them directly (the `subdomains` field below) rather than
   them being discovered like the other providers.
 - Only one A record, one AAAA record, and one TXT record exist per domain,
   always at the domain's own apex — DuckDNS has no concept of sub-records
@@ -23,7 +23,7 @@ account-wide token. Its API is much smaller than a full DNS provider's:
 class DuckDns extends DnsApi{
 	static _keyMap = {
 		token: {isRequired: true, type: 'string', isPrivate: true, displayName: 'Token'},
-		domains: {isRequired: true, type: 'string', displayName: 'Domains (comma-separated, e.g. "myhost,myhost2")'},
+		subdomains: {isRequired: true, type: 'string', displayName: 'Subdomains (comma-separated, e.g. "myhost,myhost2")'},
 	}
 
 	static displayName = 'DuckDNS';
@@ -38,7 +38,7 @@ class DuckDns extends DnsApi{
 	constructor(args){
 		super()
 		this.token = args.token;
-		this.domains = args.domains;
+		this.subdomains = args.subdomains;
 	}
 
 	// DuckDNS has one endpoint for everything: setting ip/ipv6 updates the
@@ -61,7 +61,7 @@ class DuckDns extends DnsApi{
 	// — the same thing an operator would need to do anyway when pointing a
 	// fresh DuckDNS domain at this proxy.
 	async listDomains(){
-		let labels = this.domains.split(',').map(d => d.trim()).filter(Boolean);
+		let labels = this.subdomains.split(',').map(d => d.trim()).filter(Boolean);
 		await this.update(labels.join(','), {});
 
 		return labels.map(label => ({domain: `${label}.duckdns.org`}));
diff --git a/nodejs/test/integration/dns_provider.test.js b/nodejs/test/integration/dns_provider.test.js
index 827e0a8..3196ab0 100644
--- a/nodejs/test/integration/dns_provider.test.js
+++ b/nodejs/test/integration/dns_provider.test.js
@@ -158,7 +158,7 @@ describe('DNS Provider Contract Compliance', () => {
 		const DuckDns = require('../../models/dns_provider/duckdns');
 
 		test('should meet DNS provider contract', () => {
-			const mockCredentials = {token: 'mock-token', domains: 'mockhost'};
+			const mockCredentials = {token: 'mock-token', subdomains: 'mockhost'};
 			const instance = validateDnsProviderContract(DuckDns, mockCredentials);
 
 			assert.ok(instance, 'DuckDNS provider should be instantiated');
@@ -169,27 +169,27 @@ describe('DNS Provider Contract Compliance', () => {
 			assert.strictEqual(DuckDns._keyMap.token.type, 'string');
 			assert.strictEqual(DuckDns._keyMap.token.isRequired, true);
 			assert.strictEqual(DuckDns._keyMap.token.isPrivate, true);
-			assert.ok(DuckDns._keyMap.domains, 'Should require domains');
-			assert.strictEqual(DuckDns._keyMap.domains.isRequired, true);
+			assert.ok(DuckDns._keyMap.subdomains, 'Should require subdomains');
+			assert.strictEqual(DuckDns._keyMap.subdomains.isRequired, true);
 		});
 
 		test('should have valid method signatures', () => {
-			const instance = new DuckDns({token: 'mock-token', domains: 'mockhost'});
+			const instance = new DuckDns({token: 'mock-token', subdomains: 'mockhost'});
 			validateMethodSignatures(instance);
 		});
 
 		test('should validate key mapping', () => {
-			const instance = new DuckDns({token: 'mock-token', domains: 'mockhost'});
+			const instance = new DuckDns({token: 'mock-token', subdomains: 'mockhost'});
 			validateKeyMapping(instance);
 		});
 
 		test('should validate type checking', () => {
-			const instance = new DuckDns({token: 'mock-token', domains: 'mockhost'});
+			const instance = new DuckDns({token: 'mock-token', subdomains: 'mockhost'});
 			validateTypeChecking(instance);
 		});
 
 		test('rejects non A/AAAA/TXT record creation with a clear error', async () => {
-			const instance = new DuckDns({token: 'mock-token', domains: 'mockhost'});
+			const instance = new DuckDns({token: 'mock-token', subdomains: 'mockhost'});
 			await assert.rejects(
 				() => instance.createRecord({domain: 'mockhost.duckdns.org'}, {type: 'CNAME', data: 'example.com'}),
 				/DuckDNS only supports A, AAAA and TXT records/
@@ -197,12 +197,43 @@ describe('DNS Provider Contract Compliance', () => {
 		});
 
 		test('__label strips the .duckdns.org suffix', () => {
-			const instance = new DuckDns({token: 'mock-token', domains: 'mockhost'});
+			const instance = new DuckDns({token: 'mock-token', subdomains: 'mockhost'});
 			assert.strictEqual(instance.__label({domain: 'mockhost.duckdns.org'}), 'mockhost');
 		});
 	});
 });
 
+describe('DNS Provider _keyMap field names', () => {
+	// Regression test for the DuckDNS `domains` field bug: DnsProvider merges
+	// `{...DnsProvider._keyMap, ...Provider._keyMap}` (see __intraModel in
+	// ../../models/dns_provider.js), so a provider field with the same name as
+	// one of DnsProvider's own (created_by, updated_by, name, dnsProvider,
+	// domains, id — see DnsProvider._keyMap) silently overwrites it. That
+	// happened with a DuckDNS field named `domains`, which replaced the
+	// `domains` relation (rel: 'many' to Domain) and broke updateDomains()
+	// with "this.domains.map is not a function". Every registered provider's
+	// _keyMap must avoid these names.
+	//
+	// This checks each provider class directly rather than going through
+	// ../../models/dns_provider.js, which pulls in the Redis-backed Table
+	// base class (model-redis) — not needed for a static field-name check,
+	// and not guaranteed to have a reachable Redis in every test environment.
+	const RESERVED = ['created_by', 'updated_by', 'name', 'dnsProvider', 'domains', 'id'];
+	const providerClasses = {
+		Cloudflare: require('../../models/dns_provider/cloudflare'),
+		DigitalOcean: require('../../models/dns_provider/digitalocean'),
+		PorkBun: require('../../models/dns_provider/porkbun'),
+		DuckDns: require('../../models/dns_provider/duckdns'),
+	};
+
+	for(const [name, ProviderClass] of Object.entries(providerClasses)){
+		test(`${name} does not define a reserved DnsProvider field`, () => {
+			const collisions = Object.keys(ProviderClass._keyMap).filter(key => RESERVED.includes(key));
+			assert.deepStrictEqual(collisions, [], `${name} redefines reserved field(s): ${collisions.join(', ')}`);
+		});
+	}
+});
+
 /**
  * Example: How to add tests for a new DNS provider
  *