diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index 21d4607e7cf..a002143108f 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -3013,8 +3013,11 @@ completion-file-name-table
                                        (substring string 1)
                                        pred action))
        ((eq (car-safe action) 'boundaries)
-        (let ((start (length (file-name-directory string)))
-              (end (string-search "/" (cdr action))))
+        (let ((start (length
+                      (if (string-match-p "\\`/[-[:alnum:]]+:[^:]*\\'" string)
+                          string
+                        (file-name-directory string))))
+              (end (string-search "/\\:" (cdr action))))
           `(boundaries
             ;; if `string' is "C:" in w32, (file-name-directory string)
             ;; returns "C:/", so `start' is 3 rather than 2.
@@ -4205,6 +4208,9 @@ completion-flex--make-flex-pattern
 (defun completion-flex-try-completion (string table pred point)
   "Try to flex-complete STRING in TABLE given PRED and POINT."
   (unless (and completion-flex-nospace (string-search " " string))
+    (if (and (string-match-p "\\`/[-[:alnum:]]+:[^:]*\\'" string)
+             (eq table 'completion-file-name-table))
+        (completion-basic-try-completion string table pred point)
     (pcase-let ((`(,all ,pattern ,prefix ,suffix ,_carbounds)
                  (completion-substring--all-completions
                   string table pred point
@@ -4217,7 +4223,7 @@ completion-flex-try-completion
       ;; contain the substring "config".  FIXME: this still won't
       ;; augment "foo" to "froo" when matching "frodo" and
       ;; "farfromsober".
-      (completion-pcm--merge-try pattern all prefix suffix))))
+      (completion-pcm--merge-try pattern all prefix suffix)))))

 (defun completion-flex-all-completions (string table pred point)
   "Get flex-completions of STRING in TABLE, given PRED and POINT."