diff --git a/pkgs/racket-doc/scribblings/reference/windows-paths.scrbl b/pkgs/racket-doc/scribblings/reference/windows-paths.scrbl
index c6dd5c6043..0638d1fe61 100644
--- a/pkgs/racket-doc/scribblings/reference/windows-paths.scrbl
+++ b/pkgs/racket-doc/scribblings/reference/windows-paths.scrbl
@@ -3,6 +3,8 @@
 
 @(define MzAdd (italic "Racket-specific:"))
 
+@(define (litchar~ s) (litchar (regexp-replace* "~" s " ")))
+
 @title[#:tag "windowspaths"]{Windows Paths}
 
 In general, a Windows pathname consists of an optional drive specifier
@@ -249,8 +251,8 @@ Windows paths are @techlink{cleanse}d as follows: In paths that start
 @litchar{\\?\}, redundant @litchar{\}s are removed, an extra
 @litchar{\} is added in a @litchar{\\?\REL} if an extra one is
 not already present to separate up-directory indicators from literal
-path elements, and an extra @litchar{\} is similarly added after
-@litchar{\\?\RED} if an extra one is not already present. 
+path elements, and an extra @litchar{\} is removed after
+@litchar{\\?\RED} if an extra one is present.
 @;{>> I don't know what was meant to go in place of "???", and I can't
 figure out an example that could trigger this case: <<
 When @litchar{\\?\} acts as the drive and the path contains ???, two
@@ -310,8 +312,8 @@ already. Otherwise, if no indicators or redundant separators are in
 For @racket[(split-path _path)] producing @racket[_base],
 @racket[_name], and @racket[_must-be-dir?], splitting a path that does
 not start with @litchar{\\?\} can produce parts that start with
-@litchar{\\?\}. For example, splitting @litchar{C:/x~/aux/}
-produces @litchar{\\?\C:\x~\} and @litchar{\\?\REL\\aux};
+@litchar{\\?\}. For example, splitting @litchar~{C:/x~/aux/} twice
+produces @litchar~{\\?\REL\\x~} and @litchar{\\?\REL\\aux};
 the @litchar{\\?\} is needed in these cases to preserve a
 trailing space after @litchar{x} and to avoid referring to the AUX
 device instead of an @filepath{aux} file.
diff --git a/pkgs/racket-test-core/tests/racket/path.rktl b/pkgs/racket-test-core/tests/racket/path.rktl
index bef3a08741..e30f5e2c35 100644
--- a/pkgs/racket-test-core/tests/racket/path.rktl
+++ b/pkgs/racket-test-core/tests/racket/path.rktl
@@ -638,9 +638,12 @@
   (test (string->path "\\") build-path (coerce "\\\\?\\RED\\a") 'up 'same)
 
   (test (string->path "\\") build-path (coerce "\\\\?\\RED\\..") 'up)
-  (test (string->path "\\\\?\\RED\\\\..\\") build-path (coerce "\\\\?\\RED\\\\..") (coerce "\\\\?\\RED\\\\..") 'up)
-  (test (string->path "\\\\?\\RED\\\\x\\y\\..") build-path (coerce "/x/y") (coerce "\\\\?\\RED\\.."))
-  (test (string->path "\\\\?\\c:\\x\\y\\..") build-path (coerce "c:x/y") (coerce "\\\\?\\RED\\.."))
+  (test (string->path "\\\\?\\RED\\\\..\\") build-path (coerce "\\\\?\\RED\\\\..") (coerce "\\\\?\\REL\\\\..") 'up)
+  (test (string->path "\\\\?\\RED\\\\x\\y\\..") build-path (coerce "/x/y") (coerce "\\\\?\\REL\\\\.."))
+  (test (string->path "\\\\?\\c:\\x\\y\\..") build-path (coerce "c:x/y") (coerce "\\\\?\\REL\\\\.."))
+
+  (err/rt-test (build-path (coerce "\\\\?\\RED\\\\..") (coerce "\\\\?\\RED\\\\..") 'up))
+  (err/rt-test (build-path (coerce "/x/y") (coerce "\\\\?\\RED\\..")))
 
   (test-values (list (string->path "\\\\?\\RED\\..\\")
                      (string->path "\\\\?\\REL\\\\a")
@@ -693,8 +696,10 @@
   (test (string->path "\\\\?\\UNC\\goo\\bar\\b") build-path (coerce "\\\\?\\UNC\\goo\\bar") (coerce "\\b"))
   (test (string->path "\\\\?\\\\\\b") build-path (coerce "\\\\?\\") (coerce "\\b"))
   (test (string->path "\\\\?\\\\\\b\\") build-path (coerce "\\\\?\\") (coerce "\\b\\"))
-  (err/rt-test (build-path "\\\\?\\c:" (coerce "\\b")) exn:fail:contract?)
-  
+  (err/rt-test (build-path (coerce "\\\\?\\c:") (coerce "\\b")) exn:fail:contract?)
+
+  (err/rt-test (build-path (coerce "a") (coerce "\\b")) exn:fail:contract?)
+
   ;; Don't allow path addition on bad \\?\ to change the root:
   (test (string->path "\\\\?\\\\\\c") build-path (coerce "\\\\?\\") (coerce "c"))
   (test (string->path "\\\\?\\\\\\c:") build-path (coerce "\\\\?\\") (coerce "\\\\?\\REL\\c:"))
diff --git a/racket/src/io/path/build.rkt b/racket/src/io/path/build.rkt
index e1bb5e969d..83ff0f47df 100644
--- a/racket/src/io/path/build.rkt
+++ b/racket/src/io/path/build.rkt
@@ -115,9 +115,9 @@
           (define (combine is-rel? is-complete? is-drive?)
             (when (or is-complete?
                       (and (not is-rel?)
-                           (not first?)
-                           (not (and (null? (cdr accum))
-                                     (drive? (car accum))))))
+                           (or (not first?)
+                               (not (and (null? (cdr accum))
+                                         (drive? (car accum)))))))
               (define what (if is-drive? "drive" "absolute path"))
               (raise-arguments-error who
                                      (string-append what " cannot be added to a base path")
@@ -239,7 +239,8 @@
     [(parse-unc s 0)
      => (lambda (drive-len) (just-separators-after? s drive-len))]
     [(letter-drive-start? s (bytes-length s))
-     (just-separators-after? s 2)]))
+     (just-separators-after? s 2)]
+    [else #f]))
 
 (struct starting-point (kind        ; 'rel, 'red, 'unc, or 'abs
                         bstr        ; byte string that contains the starting path
diff --git a/racket/src/racket/src/file.c b/racket/src/racket/src/file.c
index f9fe3eb839..8acbd6a395 100644
--- a/racket/src/racket/src/file.c
+++ b/racket/src/racket/src/file.c
@@ -2228,7 +2228,7 @@ static Scheme_Object *do_build_path(int argc, Scheme_Object **argv, int idelta,
 	  if (check_dos_slashslash_qm(next, len, &drive_end, NULL, &plus_sep)) {
 	    if (drive_end < 0) {
 	      /* \\?\REL\ or \\?\RED\ path */
-	      rel = 1;
+	      rel = (next[6] == 'L');
 	      is_drive = 0;
 	      if (i) {
 		int dots_end, lit_start;