Error when rendering WFFM form

I came across an interesting WFFM exception on a production CM environment today. It turned out to be a configuration error, so I decided to share

[InvalidOperationException: folder]
   Sitecore.Form.Core.Configuration.ThemesManager.GetThemeName(Item form, ID fieldID) +434
   Sitecore.Form.Core.Configuration.ThemesManager.GetThemeUrl(Item form, Boolean deviceDependant) +270
   Sitecore.Form.Core.Configuration.ThemesManager.ScriptsTags(Item form, Item contextItem) +49
   Sitecore.Form.Core.Configuration.ThemesManager.RegisterCssScript(Page page, Item form, Item contextItem) +184
   Sitecore.Form.Web.UI.Controls.SitecoreSimpleFormAscx.OnInit(EventArgs e) +233
   System.Web.UI.Control.InitRecursive(Control namingContainer) +186
   System.Web.UI.Control.AddedControl(Control control, Int32 index) +189
   Sitecore.Form.Core.Renderings.FormRender.OnInit(EventArgs e) +846
   System.Web.UI.Control.InitRecursive(Control namingContainer) +186
   System.Web.UI.Control.InitRecursive(Control namingContainer) +291
   System.Web.UI.Control.InitRecursive(Control namingContainer) +291
   System.Web.UI.Control.InitRecursive(Control namingContainer) +291
   System.Web.UI.Control.InitRecursive(Control namingContainer) +291
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2098

The method - Sitecore.Form.Core.Configuration.ThemesManager.GetThemeName(Item form, ID fieldID) - looks at the Form ID that's configured as the Forms root ID in the site definition.

string formsRootForSite = SiteUtils.GetFormsRootForSite(Context.Site);
Item item = form;
if (form.TemplateID != IDs.FormFolderTemplateID)
{
    item = form.Database.GetItem(formsRootForSite);
}
Assert.IsNotNull(item, "folder");

In my case, the configured ID did not match the actual forms folder item ID in Sitecore.