diff --git a/script/keithley.py b/script/keithley.py
index ec4484dd..fbf82b26 100644
--- a/script/keithley.py
+++ b/script/keithley.py
@@ -138,7 +138,7 @@ class Keithley(object):
             try:
                 v = int(-math.log10(value / 2)) - 1
                 v = max(v, 0)
-                v = min(v, len(RANGE_STATES)-1)
+                v = min(v, len(self.RANGE_STATES)-1)
             except ValueError:
                 v = 0
         elif isinstance(value, str):