Some of my pages won't preview in the editor

This might not be related directly to coderedcms, but I've subclassed one of the base pages from that project so maybe.

Anyhow, this is my page model:
```
class ArticleIndexPage(CoderedArticleIndexPage):
"""
Shows a list of article sub-pages.
"""

show_cover = models.BooleanField(default=False)

class Meta:
verbose_name = "Article Landing Page"

index_query_pagemodel = "cms.ArticlePage"
template = "coderedcms/pages/article_index_page.html"

body = StreamField(
LOCAL_LAYOUT_STREAMBLOCKS,
null=True,
blank=True,
use_json_field=True,
)

content_panels = CoderedArticleIndexPage.content_panels + [FieldPanel("show_cover")]
```
This page is similar to lots of other pages I've subclassed from coderedcms base pages, but this one won't generate a preview while editing in the Wagtail admin. It says there's a validation error, but there are no red dots indicating anything wrong on the page. I've tried to comment out LOCAL_LAYOUT_STREAMBLOCKS to see if that caused the issues, but it didn't change anything.

Appreciate any tips on how to solve this.