diff --git a/tsplib95/fields.py b/tsplib95/fields.py
index 7de9b88..8b1f7eb 100644
--- a/tsplib95/fields.py
+++ b/tsplib95/fields.py
@@ -369,7 +369,7 @@ def render(self, tours):
                 tour_string = ' '.join(str(i) for i in tour)
                 tour_strings.append(f'{tour_string} -1')
 
-        if tour_strings:
+        if len(tour_strings) > 1:
             tour_strings += ['-1']
 
         return '\n'.join(tour_strings)