diff --git a/diff.c b/diff.c
index 4bc9df7..0abab4a 100644
--- a/diff.c
+++ b/diff.c
@@ -4875,9 +4875,11 @@ static int diff_opt_unified(const struct option *opt,
 
 	BUG_ON_OPT_NEG(unset);
 
-	options->context = strtol(arg, &s, 10);
-	if (*s)
-		return error(_("%s expects a numerical value"), "--unified");
+	if (arg) {
+		options->context = strtol(arg, &s, 10);
+		if (*s)
+			return error(_("%s expects a numerical value"), "--unified");
+	}
 	enable_patch_output(&options->output_format);
 
 	return 0;
@@ -4895,7 +4897,7 @@ static void prep_parse_options(struct diff_options *options)
 			  DIFF_FORMAT_PATCH, DIFF_FORMAT_NO_OUTPUT),
 		OPT_CALLBACK_F('U', "unified", options, N_("<n>"),
 			       N_("generate diffs with <n> lines context"),
-			       PARSE_OPT_NONEG, diff_opt_unified),
+			       PARSE_OPT_NONEG | PARSE_OPT_OPTARG, diff_opt_unified),
 		OPT_BOOL('W', "function-context", &options->flags.funccontext,
 			 N_("generate diffs with <n> lines context")),
 		OPT_BIT_F(0, "raw", &options->output_format,