diff --git a/wfdb/io/_header.py b/wfdb/io/_header.py
index dbcc0177..419fb1cf 100644
--- a/wfdb/io/_header.py
+++ b/wfdb/io/_header.py
@@ -384,6 +384,7 @@ def _auto_signal_file_names(self):
                     num_groups += 1
                     channels_in_group = 0
             group_number.append(num_groups)
+            channels_in_group += 1
             prev_fmt = ch_fmt
             prev_spf = ch_spf