Autoincrement value of a custom field in a PostFunction Groovy ScriptRunner script

import com.atlassian.jira.ComponentManager
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.issue.MutableIssue
import com.atlassian.jira.issue.customfields.CustomFieldType
import com.atlassian.jira.issue.CustomFieldManager
import com.atlassian.jira.issue.fields.CustomField
import com.atlassian.jira.issue.ModifiedValue
import com.atlassian.jira.issue.util.DefaultIssueChangeHolder
import com.atlassian.jira.issue.util.IssueChangeHolder

ComponentManager componentManager = ComponentManager.getInstance()
CustomFieldManager customFieldManager = componentManager.getCustomFieldManager()
CustomField cf = customFieldManager.getCustomFieldObject("customfield_18126")
Double val = (issue.getCustomFieldValue(cf) as Double)
if ( val == null) 
 val = 1
else
 val = val + 1
IssueChangeHolder changeHolder = new DefaultIssueChangeHolder();
cf.updateValue(null, issue, new ModifiedValue(issue.getCustomFieldValue(cf),val), changeHolder)