diff --git a/src/calibre/gui2/dialogs/metadata_single.py b/src/calibre/gui2/dialogs/metadata_single.py
index 7a8e4ea8d0..fa20658c12 100644
--- a/src/calibre/gui2/dialogs/metadata_single.py
+++ b/src/calibre/gui2/dialogs/metadata_single.py
@@ -429,10 +429,12 @@ class MetadataSingleDialog(ResizableDialog, Ui_MetadataSingleDialog):
                 old_extensions.add(ext)
         for ext in new_extensions:
             self.db.add_format(self.row, ext, open(paths[ext], 'rb'), notify=False)
-        db_extensions = set([f.lower() for f in self.db.formats(self.row).split(',')])
+        dbfmts = self.db.formats(self.row)
+        db_extensions = set([f.lower() for f in (dbfmts.split(',') if dbfmts
+            else [])])
         extensions = new_extensions.union(old_extensions)
         for ext in db_extensions:
-            if ext not in extensions:
+            if ext not in extensions and ext in self.original_formats:
                 self.db.remove_format(self.row, ext, notify=False)
 
     def show_format(self, item, *args):
@@ -576,6 +578,7 @@ class MetadataSingleDialog(ResizableDialog, Ui_MetadataSingleDialog):
         self.orig_date = qt_to_dt(self.date.date())
 
         exts = self.db.formats(row)
+        self.original_formats = []
         if exts:
             exts = exts.split(',')
             for ext in exts:
@@ -586,6 +589,7 @@ class MetadataSingleDialog(ResizableDialog, Ui_MetadataSingleDialog):
                 if size is None:
                     continue
                 Format(self.formats, ext, size, timestamp=timestamp)
+                self.original_formats.append(ext.lower())
 
 
         self.initialize_combos()
diff --git a/src/calibre/gui2/metadata/basic_widgets.py b/src/calibre/gui2/metadata/basic_widgets.py
index 590a8be3bb..d3fa5958ab 100644
--- a/src/calibre/gui2/metadata/basic_widgets.py
+++ b/src/calibre/gui2/metadata/basic_widgets.py
@@ -472,6 +472,7 @@ class FormatsManager(QWidget): # {{{
     def initialize(self, db, id_):
         self.changed = False
         exts = db.formats(id_, index_is_id=True)
+        self.original_val = set([])
         if exts:
             exts = exts.split(',')
             for ext in exts:
@@ -482,6 +483,7 @@ class FormatsManager(QWidget): # {{{
                 if size is None:
                     continue
                 Format(self.formats, ext, size, timestamp=timestamp)
+                self.original_val.add(ext.lower())
 
     def commit(self, db, id_):
         if not self.changed:
@@ -500,11 +502,12 @@ class FormatsManager(QWidget): # {{{
         for ext in new_extensions:
             db.add_format(id_, ext, open(paths[ext], 'rb'), notify=False,
                     index_is_id=True)
-        db_extensions = set([f.lower() for f in db.formats(id_,
-            index_is_id=True).split(',')])
+        dbfmts = db.formats(id_, index_is_id=True)
+        db_extensions = set([f.lower() for f in (dbfmts.split(',') if dbfmts
+            else [])])
         extensions = new_extensions.union(old_extensions)
         for ext in db_extensions:
-            if ext not in extensions:
+            if ext not in extensions and ext in self.original_val:
                 db.remove_format(id_, ext, notify=False, index_is_id=True)
 
         self.changed = False