diff --git a/diff.c b/diff.c
index 56d7bfd..513410d 100644
--- a/diff.c
+++ b/diff.c
@@ -22,6 +22,7 @@
 #include "argv-array.h"
 #include "graph.h"
 #include "packfile.h"
+#include "help.h"
 
 #ifdef NO_FAST_WORKING_DIRECTORY
 #define FAST_WORKING_DIRECTORY 0
@@ -93,6 +94,8 @@ static NORETURN void die_want_option(const char *option_name)
 	die(_("option '%s' requires a value"), option_name);
 }
 
+define_list_config_array_extra(color_diff_slots, {"plain"});
+
 static int parse_diff_color_slot(const char *var)
 {
 	if (!strcasecmp(var, "plain"))