diff --git a/wfdb/io/_header.py b/wfdb/io/_header.py
index 2be39c19..374d4589 100644
--- a/wfdb/io/_header.py
+++ b/wfdb/io/_header.py
@@ -107,7 +107,7 @@
     ["(?P<file_name>~?[-\w]*\.?[\w]*)[ \t]+(?P<fmt>\d+)x?"
      "(?P<samps_per_frame>\d*):?(?P<skew>\d*)\+?(?P<byte_offset>\d*)[ \t]*",
      "(?P<adc_gain>-?\d*\.?\d*e?[\+-]?\d*)\(?(?P<baseline>-?\d*)\)?",
-     "/?(?P<units>[\w\^\-\?%]*)[ \t]*(?P<adc_res>\d*)[ \t]*",
+     "/?(?P<units>[\w\^\-\?%\/]*)[ \t]*(?P<adc_res>\d*)[ \t]*",
      "(?P<adc_zero>-?\d*)[ \t]*(?P<init_value>-?\d*)[ \t]*(?P<checksum>-?\d*)",
      "[ \t]*(?P<block_size>\d*)[ \t]*(?P<sig_name>[\S]?[^\t\n\r\f\v]*)"])
 )