diff --git a/lisp/faces.el b/lisp/faces.el
index c3693d16631d..e073ed266c0c 100644
--- a/lisp/faces.el
+++ b/lisp/faces.el
@@ -2465,9 +2465,12 @@ If you set `term-file-prefix' to nil, this function does nothing."
   :version "21.1"
   :group 'basic-faces)
 
-;; Definition stolen from linum.el.
+;; Definition originally stolen from linum.el.
+;; The monospace part is so we don't accidentally display numbers
+;; using a variable-pitch font just because the default face uses
+;; such a font.
 (defface line-number
-  '((t :inherit (shadow default)))
+  '((t :inherit (shadow default) :family "Monospace Serif"))
   "Face for displaying line numbers.
 This face is used when `display-line-numbers' is non-nil.