diff --git a/lib/internal/http.js b/lib/internal/http.js
index 4f250a2e70a20f..aa3ec354dabf4a 100644
--- a/lib/internal/http.js
+++ b/lib/internal/http.js
@@ -112,7 +112,9 @@ class ProxyConfig {
     const { host, hostname, port, protocol, username, password } = new URL(proxyUrl);
     this.href = proxyUrl; // Full URL of the proxy server.
     this.host = host; // Full host including port, e.g. 'localhost:8080'.
-    this.hostname = hostname.replace(/^\[|\]$/g, ''); // Trim off the brackets from IPv6 addresses.
+    // Trim off the brackets from IPv6 addresses. As it's parsed from a valid URL, an opening
+    // "[" Must already have a matching "]" at the end.
+    this.hostname = hostname[0] === '[' ? hostname.slice(1, -1) : hostname;
     this.port = port ? NumberParseInt(port, 10) : (protocol === 'https:' ? 443 : 80);
     this.protocol = protocol; // Protocol of the proxy server, e.g. 'http:' or 'https:'.