diff --git a/packages/backend/src/core/UtilityService.ts b/packages/backend/src/core/UtilityService.ts
index 81eaa5f95d..9ed8138ffe 100644
--- a/packages/backend/src/core/UtilityService.ts
+++ b/packages/backend/src/core/UtilityService.ts
@@ -106,13 +106,22 @@ export class UtilityService {
 
 	@bindThis
 	public toPuny(host: string): string {
-		return domainToASCII(host.toLowerCase());
+		// domainToASCII will return an empty string if we give it a
+		// string like `name:123`, but `host` may well be in that form
+		// (e.g. when testing locally, you'll get `localhost:3000`); split
+		// the port off, and add it back later
+		const hostParts = host.toLowerCase().match(/^(.+?)(:.+)?$/);
+		if (!hostParts) return '';
+		const hostname = hostParts[1];
+		const port = hostParts[2] ?? '';
+
+		return domainToASCII(hostname) + port;
 	}
 
 	@bindThis
 	public toPunyNullable(host: string | null | undefined): string | null {
 		if (host == null) return null;
-		return domainToASCII(host.toLowerCase());
+		return this.toPuny(host);
 	}
 
 	@bindThis
diff --git a/packages/backend/test/unit/UtilityService.ts b/packages/backend/test/unit/UtilityService.ts
index d86e794f2f..cb010ff1f9 100644
--- a/packages/backend/test/unit/UtilityService.ts
+++ b/packages/backend/test/unit/UtilityService.ts
@@ -22,6 +22,12 @@ describe('UtilityService', () => {
 		test('japanese', () => {
 			assert.equal(utilityService.punyHost('http://www.新聞.com'), 'www.xn--efvv70d.com');
 		});
+		test('simple, with port', () => {
+			assert.equal(utilityService.punyHost('http://www.foo.com:3000'), 'www.foo.com:3000');
+		});
+		test('japanese, with port', () => {
+			assert.equal(utilityService.punyHost('http://www.新聞.com:3000'), 'www.xn--efvv70d.com:3000');
+		});
 	});
 
 	describe('punyHostPSLDomain', () => {
@@ -31,6 +37,12 @@ describe('UtilityService', () => {
 		test('japanese', () => {
 			assert.equal(utilityService.punyHostPSLDomain('http://www.新聞.com'), 'xn--efvv70d.com');
 		});
+		test('simple, with port', () => {
+			assert.equal(utilityService.punyHostPSLDomain('http://www.foo.com:3000'), 'foo.com:3000');
+		});
+		test('japanese, with port', () => {
+			assert.equal(utilityService.punyHostPSLDomain('http://www.新聞.com:3000'), 'xn--efvv70d.com:3000');
+		});
 		test('lower', () => {
 			assert.equal(utilityService.punyHostPSLDomain('http://foo.github.io'), 'foo.github.io');
 			assert.equal(utilityService.punyHostPSLDomain('http://foo.bar.github.io'), 'bar.github.io');
@@ -40,4 +52,13 @@ describe('UtilityService', () => {
 			assert.equal(utilityService.punyHostPSLDomain('http://foo.bar.masto.host'), 'bar.masto.host');
 		});
 	});
+
+	describe('toPuny', () => {
+		test('without port ', () => {
+			assert.equal(utilityService.toPuny('www.foo.com'), 'www.foo.com');
+		});
+		test('with port ', () => {
+			assert.equal(utilityService.toPuny('www.foo.com:3000'), 'www.foo.com:3000');
+		});
+	});
 });