second changeset in the Change Log not working

I see this error when trying to add a column to a table that already exists.

Unexpected error running Liquibase: Table 'DATABASECHANGELOG' already exists

The ChangeLog file, that i am executing from the Command prompt is :

<?xml version="1.0" encoding="UTF-8"?>

<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">

<changeSet id="1.0" author="Jitu">
<createTable tableName="student">
<column name="id" type="int">
</column>
<column name="name" type="varchar(50)">
</column>
<column name="enrolled" type="boolean"/>
</createTable>
</changeSet>
<changeSet id="2.0" author="Jitu">
<addColumn tableName="student">
<column name="grade" type="float(7,2)">
</column>
</addColumn>
</changeSet>
</databaseChangeLog>

ChangeSet id 2.0 is giving me this issue.

Please help!