diff --git a/diff.c b/diff.c
index 0391e8c42..0f839c11b 100644
--- a/diff.c
+++ b/diff.c
@@ -853,6 +853,10 @@ static int parse_num(const char **cp_p)
 	}
 	*cp_p = cp;
 
+	/* special case: -M100 would mean 1.0 not 0.1 */
+	if (num == 100 && scale == 1000)
+		return MAX_SCORE;
+
 	/* user says num divided by scale and we say internally that
 	 * is MAX_SCORE * num / scale.
 	 */
diff --git a/diffcore-rename.c b/diffcore-rename.c
index 6a9d95d05..dba965c0b 100644
--- a/diffcore-rename.c
+++ b/diffcore-rename.c
@@ -307,6 +307,9 @@ void diffcore_rename(struct diff_options *options)
 	if (rename_count == rename_dst_nr)
 		goto cleanup;
 
+	if (minimum_score == MAX_SCORE)
+		goto cleanup;
+
 	num_create = (rename_dst_nr - rename_count);
 	num_src = rename_src_nr;
 	mx = xmalloc(sizeof(*mx) * num_create * num_src);