diff --git a/src/macfont.m b/src/macfont.m
index e3dab1c42e0..b30c3c9763c 100644
--- a/src/macfont.m
+++ b/src/macfont.m
@@ -2628,6 +2628,9 @@ So we use CTFontDescriptorCreateMatchingFontDescriptor (no
   font->pixel_size = size;
   font->driver = &macfont_driver;
   font->encoding_charset = font->repertory_charset = -1;
+  /* Clear font->space_width so macfont_monospace_width_multiplier may
+     not be confused by an uninitialized value.  */
+  font->space_width = 0;
 
   block_input ();