diff --git a/lisp/cedet/semantic/decorate/mode.el b/lisp/cedet/semantic/decorate/mode.el
index 27634102df9..6271fb1ced6 100644
--- a/lisp/cedet/semantic/decorate/mode.el
+++ b/lisp/cedet/semantic/decorate/mode.el
@@ -409,9 +409,11 @@ decoration API found in this library."
        ;; Create an override method to specify if a given tag belongs
        ;; to this type of decoration
        (define-overloadable-function ,predicate (tag)
-         ,(internal--format-docstring-line
-           "Return non-nil to decorate TAG with `%s' style.\n%s"
-           name doc))
+         ,(concat
+           (internal--format-docstring-line
+            "Return non-nil to decorate TAG with `%s' style."
+            name)
+           "\n" doc))
        ;; Create an override method that will perform the highlight
        ;; operation if the -p method returns non-nil.
        (define-overloadable-function ,highlighter (tag)
diff --git a/lisp/emacs-lisp/easy-mmode.el b/lisp/emacs-lisp/easy-mmode.el
index 0e4d6eeb7bb..d37bca24a0f 100644
--- a/lisp/emacs-lisp/easy-mmode.el
+++ b/lisp/emacs-lisp/easy-mmode.el
@@ -496,15 +496,17 @@ on if the hook has explicitly disabled it.
        (define-minor-mode ,global-mode
          ,(concat (format "Toggle %s in all buffers.\n" pretty-name)
                   (internal--format-docstring-line
-                   "With prefix ARG, enable %s if ARG is positive; otherwise, \
-disable it.\n\n"
+                   (concat "With prefix ARG, enable %s if ARG is positive; "
+                           "otherwise, disable it.")
                    pretty-global-name)
+                  "\n\n"
                   "If called from Lisp, toggle the mode if ARG is `toggle'.
 Enable the mode if ARG is nil, omitted, or is a positive number.
 Disable the mode if ARG is a negative number.\n\n"
                   (internal--format-docstring-line
-                   "%s is enabled in all buffers where `%s' would do it.\n\n"
+                   "%s is enabled in all buffers where `%s' would do it."
                    pretty-name turn-on)
+                  "\n\n"
                   (internal--format-docstring-line
                    "See `%s' for more information on %s."
                    mode pretty-name)
diff --git a/lisp/subr.el b/lisp/subr.el
index 8cb79b290b7..1d2980802e0 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -6435,8 +6435,11 @@ of fill.el (for example `fill-region')."
       (concat fst (internal--fill-string-single-line lst)))))
 
 (defun internal--format-docstring-line (string &rest objects)
-  "Format a documentation string out of STRING and OBJECTS.
+  "Format a single line from a documentation string out of STRING and OBJECTS.
+Signal an error if STRING contains a newline.
 This is intended for internal use only."
+  (when (string-match "\n" string)
+    (error "Unable to fill string containing newline: %S" string))
   (internal--fill-string-single-line (apply #'format string objects)))
 
 (defun json-available-p ()