diff --git a/Tools/autotest/param_metadata/param.py b/Tools/autotest/param_metadata/param.py
index 8120e0ad77..deb7861c42 100644
--- a/Tools/autotest/param_metadata/param.py
+++ b/Tools/autotest/param_metadata/param.py
@@ -124,6 +124,7 @@ known_units = {
              'RPM'     : 'Revolutions Per Minute',
              'kg/m/m'  : 'kilograms per square meter', # metre is the SI unit name, meter is the american spelling of it
              'kg/m/m/m': 'kilograms per cubic meter',
+             'litres'  : 'litres',
              }
 
 required_param_fields = [