diff --git a/index.js b/index.js
index 47ae24c..c8a4077 100644
--- a/index.js
+++ b/index.js
@@ -12,8 +12,10 @@ const supportedProtocols = new Set([
 
 const hasCustomProtocol = urlString => {
 	try {
+		const regex = /\./;
 		const {protocol} = new URL(urlString);
-		return protocol.endsWith(':') && !supportedProtocols.has(protocol);
+
+		return protocol.endsWith(':') && !regex.test(protocol) && !supportedProtocols.has(protocol);
 	} catch {
 		return false;
 	}
diff --git a/test.js b/test.js
index 4c74a88..176cdcf 100644
--- a/test.js
+++ b/test.js
@@ -201,6 +201,7 @@ test('removeExplicitPort option', t => {
 	t.is(normalizeUrl('https://sindresorhus.com:123', options), 'https://sindresorhus.com');
 	t.is(normalizeUrl('http://sindresorhus.com:443', options), 'http://sindresorhus.com');
 	t.is(normalizeUrl('https://sindresorhus.com:80', options), 'https://sindresorhus.com');
+	t.is(normalizeUrl('sindresorhus.com:123', options), 'http://sindresorhus.com');
 });
 
 test('removeSingleSlash option', t => {