diff --git a/merge-recursive.c b/merge-recursive.c
index 40eb840a52..dbb7104c04 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -2069,8 +2069,8 @@ int parse_merge_opt(struct merge_options *o, const char *s)
 		o->xdl_opts = DIFF_WITH_ALG(o, PATIENCE_DIFF);
 	else if (!strcmp(s, "histogram"))
 		o->xdl_opts = DIFF_WITH_ALG(o, HISTOGRAM_DIFF);
-	else if (!strcmp(s, "diff-algorithm=")) {
-		long value = parse_algorithm_value(s+15);
+	else if (!prefixcmp(s, "diff-algorithm=")) {
+		long value = parse_algorithm_value(s + strlen("diff-algorithm="));
 		if (value < 0)
 			return -1;
 		/* clear out previous settings */