J48 Weka in Knime error Cannot handle string class

Hello everyone,

This is my first time using KNIME. I'm trying to classify some tweets into 3 sentiment class: positive, negative and neutral. The data includes 2 columns: the tweet and the manually classified class. In an attempt to increase the accuracy, I used J48 (3.7) node from the KNIME node repository, but it failed with the following error:

ERROR J48 (3.7)            0:293      Execute failed: Weka classifier cannot work with given data. Reason: weka.classifiers.trees.J48: Cannot handle string class!

How can I resolve this problem with this node? The actual workflow I am working on is in the attachment and KNIME 3.1.1 is currently being used.

Thank you!