Scan of CM energy in MAdgraph

Hi,
I am wondering how to scan of CM energy in Madgraph.
When I typed it as
 scan:[500, 600, 700] for one beam and
 for other beam as
 scan:[500,600,700]
 It is showing an error given below

Command "generate_events run_01" interrupted with error:
Exception : scan:[500.0, 600.0, 700.0, 800.0, 900.0, 1000.0] can not be mapped to a float
Please report this bug on https://bugs.launchpad.net/madgraph5
More information is found in 'ME5_debug'.
Please attach this file to your report.
INFO:

May I ask you for my mistake and what will be the solution for this?

Thanking you.
Regards,
Deepanjali