diff --git a/diff.c b/diff.c
index 0724fa6ff..08e39144c 100644
--- a/diff.c
+++ b/diff.c
@@ -3634,8 +3634,8 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		options->xdl_opts = DIFF_WITH_ALG(options, PATIENCE_DIFF);
 	else if (!strcmp(arg, "--histogram"))
 		options->xdl_opts = DIFF_WITH_ALG(options, HISTOGRAM_DIFF);
-	else if (!prefixcmp(arg, "--diff-algorithm=")) {
-		long value = parse_algorithm_value(arg+17);
+	else if ((argcount = parse_long_opt("diff-algorithm", av, &optarg))) {
+		long value = parse_algorithm_value(optarg);
 		if (value < 0)
 			return error("option diff-algorithm accepts \"myers\", "
 				     "\"minimal\", \"patience\" and \"histogram\"");
@@ -3643,6 +3643,7 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		DIFF_XDL_CLR(options, NEED_MINIMAL);
 		options->xdl_opts &= ~XDF_DIFF_ALGORITHM_MASK;
 		options->xdl_opts |= value;
+		return argcount;
 	}
 
 	/* flags options */