title: Error in using tag in command line when using liquibase
body: <p>I wanted to use the liquibase tag functionality. At first, I run the following command into cmd (with liquibase.properties set up etc)</p>
  
  <pre><code>liquibase --changeLogFile=changeLog.xml update
  </code></pre>
  
  <p>where the changeLog.xml contains as follows:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="UTF-8"?&gt;
  
  &lt;databaseChangeLog
    xmlns="http://www.liquibase.org/xml/ns/dbchangelog"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog
           http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-3.1.xsd"&gt;
  
      &lt;changeSet id="1" author="ivan"&gt;
          &lt;createTable tableName="customer"&gt;
              &lt;column name="id" type="varchar2(42)" /&gt;
              &lt;column name="name" type="varchar2(42)" /&gt;
          &lt;/createTable&gt;
      &lt;/changeSet&gt;
  &lt;/databaseChangeLog&gt;
  </code></pre>
  
  <p>and no error occurs.</p>
  
  <p>Now, I want to tag the current database state, by using the following command:</p>
  
  <pre><code>liquibase --changeLogFile=changeLog.xml tag exampletag
  </code></pre>
  
  <p>and I've also tried using the following command</p>
  
  <pre><code>liquibase tag exampletag
  </code></pre>
  
  <p>alas, both attempts failed with the same error message, as follows</p>
  
  <pre><code>Unexpected error running Liquibase: Unknown Reason
  </code></pre>
  
  <p>How do I properly tag a database in the current state, by passing the commands via command line? Thanks!</p>
  
  <p><strong>UPDATE</strong></p>
  
  <p>This is the stack trace I obtained from running the first command. And also, I am using Liquibase 3.2.0</p>
  
  <pre><code>DEBUG 6/11/14 2:42 PM: liquibase: Connected to N603066@jdbc:oracle:thin:@10.111.48.125:1521:orcl
  DEBUG 6/11/14 2:42 PM: liquibase: Setting auto commit to false from true
  DEBUG 6/11/14 2:42 PM: liquibase: Computed checksum for 1402468965639 as 984dfd7dd9a2496bb4b3c74eeafa6a64
  DEBUG 6/11/14 2:42 PM: liquibase: Executing QUERY database command: select count(*) from N603066.DATABASECHANGELOGLOCK
  DEBUG 6/11/14 2:42 PM: liquibase: Executing QUERY database command: SELECT LOCKED FROM N603066.DATABASECHANGELOGLOCK WHERE ID=1 FOR UPDATE
  DEBUG 6/11/14 2:42 PM: liquibase: Lock Database
  DEBUG 6/11/14 2:42 PM: liquibase: Executing UPDATE database command: UPDATE N603066.DATABASECHANGELOGLOCK SET LOCKED = 1, LOCKEDBY = 'LD418102Y (10.111.48.204)', LOCKGRANTED = to_date('2014-06-11 14:42:45', 'YYYY-MM-DD HH24:MI:SS') WHERE ID = 1 AND LOCKED = 0
  INFO 6/11/14 2:42 PM: liquibase: Successfully acquired change log lock
  DEBUG 6/11/14 2:42 PM: liquibase: Executing QUERY database command: SELECT MD5SUM FROM N603066.DATABASECHANGELOG WHERE MD5SUM IS NOT NULL
  DEBUG 6/11/14 2:42 PM: liquibase: Executing QUERY database command: select count(*) from N603066.DATABASECHANGELOGLOCK
  DEBUG 6/11/14 2:42 PM: liquibase: Executing QUERY database command: SELECT COUNT(*) FROM N603066.DATABASECHANGELOG
  DEBUG 6/11/14 2:42 PM: liquibase: Executing EXECUTE database command: UPDATE N603066.DATABASECHANGELOG SET TAG = 'exampletag' WHERE DATEEXECUTED = (SELECT MAX(DATEEXECUTED) FROM N603066.DATABASECHANGELOG)
  DEBUG 6/11/14 2:42 PM: liquibase: Release Database Lock
  DEBUG 6/11/14 2:42 PM: liquibase: Executing UPDATE database command: UPDATE N603066.DATABASECHANGELOGLOCK SET LOCKED = 0, LOCKEDBY = NULL, LOCKGRANTED = NULL WHERE ID = 1
  INFO 6/11/14 2:42 PM: liquibase: Successfully released change log lock
  Unexpected error running Liquibase: Unknown Reason
  
  SEVERE 6/11/14 2:42 PM: liquibase: Unknown Reason
  liquibase.exception.DatabaseException: java.lang.NullPointerException
      at liquibase.changelog.StandardChangeLogHistoryService.tag(StandardChangeLogHistoryService.java:306)
      at liquibase.database.AbstractJdbcDatabase.tag(AbstractJdbcDatabase.java:834)
      at liquibase.Liquibase.tag(Liquibase.java:695)
      at liquibase.integration.commandline.Main.doMigration(Main.java:939)
      at liquibase.integration.commandline.Main.run(Main.java:170)
      at liquibase.integration.commandline.Main.main(Main.java:89)
  Caused by: java.lang.NullPointerException
      at liquibase.changelog.StandardChangeLogHistoryService.tag(StandardChangeLogHistoryService.java:304)
      ... 5 more
  
  
  For more information, use the --logLevel flag
  </code></pre>