diff --git a/src/calibre/ebooks/oeb/polish/pretty.py b/src/calibre/ebooks/oeb/polish/pretty.py
index 893fb569f6..94db83571d 100644
--- a/src/calibre/ebooks/oeb/polish/pretty.py
+++ b/src/calibre/ebooks/oeb/polish/pretty.py
@@ -106,7 +106,7 @@ def has_only_blocks(x):
     if x.text and not isspace(x.text):
         return False
     for child in x:
-        if not isblock(child) or (child.tail and isspace(child.tail)):
+        if not isblock(child) or (child.tail and not isspace(child.tail)):
             return False
     return True