diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index 5d7cbcedd41..52a286018a1 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -4025,7 +4025,9 @@ the same set of elements."
                      (unique (or (and (eq prefix t) (setq prefix fixed))
                                  (and (stringp prefix)
                                       (eq t (try-completion prefix comps))))))
-                (unless (or (eq elem 'prefix)
+                ;; if the common prefix is unique, it also is a common
+                ;; suffix, so we should add it for `prefix' elements
+                (unless (or (and (eq elem 'prefix) (not unique))
                             (equal prefix ""))
                   (push prefix res))
                 ;; If there's only one completion, `elem' is not useful
diff --git a/test/lisp/minibuffer-tests.el b/test/lisp/minibuffer-tests.el
index ff58d35eb3e..4f92d7f841c 100644
--- a/test/lisp/minibuffer-tests.el
+++ b/test/lisp/minibuffer-tests.el
@@ -298,6 +298,19 @@
                   "jab" '("dabjabstabby" "many") nil 3)))
            6)))
 
+(ert-deftest completion-substring-test-5 ()
+  ;; merge-completions needs to work correctly when
+  (should (equal
+           (completion-pcm--merge-completions '("ab" "sab") '(prefix "b"))
+           '("b" "a" prefix)))
+  (should (equal
+           (completion-pcm--merge-completions '("ab" "ab") '(prefix "b"))
+           '("b" "a")))
+  ;; substring completion should successfully complete the entire string
+  (should (equal
+           (completion-substring-try-completion "b" '("ab" "ab") nil 0)
+           '("ab" . 2))))
+
 (ert-deftest completion-flex-test-1 ()
   ;; Fuzzy match
   (should (equal