diff --git a/basis/urls/urls-docs.factor b/basis/urls/urls-docs.factor
index 707caf3188..eb8e452ca4 100644
--- a/basis/urls/urls-docs.factor
+++ b/basis/urls/urls-docs.factor
@@ -65,9 +65,8 @@ HELP: derive-url
 } ;
 
 HELP: ensure-port
-{ $values { "url" url } }
-{ $description "If the URL does not specify a port number, fill in the default for the URL's protocol. If the protocol is unknown, the port number is not changed." }
-{ $side-effects "url" }
+{ $values { "url" url } { "url'" url } }
+{ $description "If the URL does not specify a port number, create a new URL which is equal except the port number is set to the default for the URL's protocol. If the protocol is unknown, outputs an exact copy of the input URL." }
 { $examples
     { $example
         "USING: accessors prettyprint urls ;"
diff --git a/basis/urls/urls.factor b/basis/urls/urls.factor
index 38d0016d56..1e886ae3e2 100644
--- a/basis/urls/urls.factor
+++ b/basis/urls/urls.factor
@@ -175,8 +175,8 @@ PRIVATE>
     ] [ protocol>> ] bi
     secure-protocol? [ >secure-addr ] when ;
 
-: ensure-port ( url -- url )
-    dup protocol>> '[ _ protocol-port or ] change-port ;
+: ensure-port ( url -- url' )
+    clone dup protocol>> '[ _ protocol-port or ] change-port ;
 
 ! Literal syntax
 SYNTAX: URL" lexer get skip-blank parse-string >url parsed ;