diff --git a/bookwyrm/templatetags/utilities.py b/bookwyrm/templatetags/utilities.py
index bcb1ec4e..abb52499 100644
--- a/bookwyrm/templatetags/utilities.py
+++ b/bookwyrm/templatetags/utilities.py
@@ -56,6 +56,8 @@ def truncatepath(value, arg):
 @register.simple_tag(takes_context=False)
 def get_book_cover_thumbnail(book, size="medium", ext="jpg"):
     """Returns a book thumbnail at the specified size and extension, with fallback if needed"""
+    if size == "":
+        size = "medium"
     try:
         cover_thumbnail = getattr(book, "cover_bw_book_%s_%s" % (size, ext))
         return cover_thumbnail.url