diff --git a/net-doc/net/scribblings/url.scrbl b/net-doc/net/scribblings/url.scrbl
index 0fc4adceca..d6e402b455 100644
--- a/net-doc/net/scribblings/url.scrbl
+++ b/net-doc/net/scribblings/url.scrbl
@@ -171,7 +171,8 @@ scheme @racket["http"].}
 
 Generates a string corresponding to the contents of a @racket[url]
 struct.  For a @racket["file:"] URL, the URL must not be relative, and
-the result always starts @litchar{file://}.
+the result always starts @litchar{file://}. For a URL with a host,
+user, or port, its path must be either absolute or empty.
 
 The @racket[url->string] procedure uses
 @racket[alist->form-urlencoded] when formatting the query, so it is
diff --git a/net-test/tests/net/url-string.rkt b/net-test/tests/net/url-string.rkt
index a7b23d54e3..94ff1a4d68 100644
--- a/net-test/tests/net/url-string.rkt
+++ b/net-test/tests/net/url-string.rkt
@@ -91,6 +91,11 @@
   (test-s->u #("http" #f "www.drscheme.org" #f #t (#("a" "x") #("b") #("c" "b")) () #f)
              "http://www.drscheme.org/a;x/b/c;b")
 
+  ;; Allow empty port spec
+  (test #("http" #f "www.drscheme.org" #f #t (#("a")) () #f)
+        url->vec
+        (string->url "http://www.drscheme.org:/a"))
+
   ;; Test IPv6 literal addresses, which are written with "[...]" around
   ;; an address that contains ":"s:
   (test-s->u #("http" #f "::1" #f #t (#("a") #("b") #("c")) () "joe")
@@ -158,6 +163,18 @@
    (string->url "a*b://www.foo.com/") =error> url-exception?
    (string->url "a b://www.foo.com/") =error> url-exception?)
 
+  ;; test relatve path after host
+  (test
+   (string->url "http://www.foo.com:oops/") =error> url-exception?
+   (string->url "http://www.foo.com:0oops/") =error> url-exception?
+
+   ;; Empty "relative" path is ok with host:
+   (url->string (url "http" #f "www.drscheme.org" #f #f (list) (list) #f))
+   => "http://www.drscheme.org"
+   ;; Non-empty "relative" path is not ok with host:
+   (url->string (url "http" #f "www.drscheme.org" #f #f (list (path/param "a" (list))) (list) #f))
+   =error> exn:fail:contract?)
+
   ;; test file: urls
   (test-s->u #("file" #f "" #f #t (#("abc") #("def.html")) () #f)
              "file:///abc/def.html")