What does "Not PersistText value" mean?

I'm new to Yesod and I'm trying to make the same blog project from this screencast: https://www.youtube.com/watch?v=SadfV-qbVg8 with the only difference being that I'm using MariaDB instead of PostgreSQL. Every time I add a new blog post and redirect to the page that shows it, I see this error:

[Error#yesod-core] get BlogPostKey {unBlogPostKey = SqlBackendKey {unSqlBackendKey = 5}}: field article: Not a PersistText value @(yesod-core-1.4.12:Yesod.Core.Class.Yesod ./Yesod/Core/Class/Yesod.hs:577:5)

      

        
        
        
      

    

What exactly does this mean? If I look into the database, I have all the records saved correctly. Why can't it download messages from the database?

Here's the code

model

User
    ident Text
    password Text Maybe
    UniqueUser ident
    deriving Typeable
Email
    email Text
    user UserId Maybe
    verkey Text Maybe
    UniqueEmail email

BlogPost
    title Text
    article Markdown

      

        
        
        
      

    

PostDetails.hs (gets message from DB and shows it)

module Handler.PostDetails where

import Import

getPostDetailsR :: BlogPostId -> Handler Html
getPostDetailsR blogPostId = do     
    blogPost <- runDB $ get404 blogPostId 
    defaultLayout $ do
    $(widgetFile "postDetails/post")

      

        
        
        
      

    

PostNew.hs (Creates a new message and saves it to the database, after insertion, redirects it to PostDetails.hs with a new message)

module Handler.PostNew where

import Import
import Yesod.Form.Bootstrap3
import Yesod.Text.Markdown

blogPostForm :: AForm Handler BlogPost
blogPostForm = BlogPost 
            <$> areq textField (bfs ("Title" :: Text)) Nothing
            <*> areq markdownField (bfs ("Article" :: Text)) Nothing

getPostNewR :: Handler Html
getPostNewR = do
    (widget, enctype) <- generateFormPost $ renderBootstrap3 BootstrapBasicForm blogPostForm
    defaultLayout $ do
        $(widgetFile "posts/new")

postPostNewR :: Handler Html
postPostNewR = do
    ((res, widget), enctype) <- runFormPost $ renderBootstrap3 BootstrapBasicForm blogPostForm
    case res of
     FormSuccess blogPost -> do
        blogPostId <- runDB $ insert blogPost
        redirect $ PostDetailsR blogPostId
     _ -> defaultLayout $(widgetFile "posts/new")

      

        
        
        
      

    

I don't understand why the compiler didn't catch this error. When I create a post, I use "Internal Server Error" instead of the header