diff --git a/wani.py b/wani.py
index 7d10c1d..64ce126 100644
--- a/wani.py
+++ b/wani.py
@@ -1186,6 +1186,9 @@ class Writer:
             file_handler.flush()
 
     def write_out(self, structures, colocation_ids):
+        if self.output_file is None:
+            return
+
         def fp_close(fp_):
             if fp_ != sys.stdout:
                 fp_.close()