diff --git a/copyparty/mtag.py b/copyparty/mtag.py
index e006b688..8e17a712 100644
--- a/copyparty/mtag.py
+++ b/copyparty/mtag.py
@@ -131,7 +131,7 @@ class MParser(object):
 
 def au_unpk(
     log: "NamedLogger", fmt_map: dict[str, str], abspath: str, vn: Optional[VFS] = None
-) -> str:
+) -> Optional[str]:
     ret = ""
     maxsz = 1024 * 1024 * 64
     try:
@@ -178,23 +178,26 @@ def au_unpk(
 
         elif pk == "epub":
             fi = get_cover_from_epub(log, abspath)
-            assert fi  # !rm
 
         else:
             raise Exception("unknown compression %s" % (pk,))
 
-        fsz = 0
-        with os.fdopen(fd, "wb") as fo:
-            while True:
-                buf = fi.read(32768)
-                if not buf:
-                    break
+        if fi:
+            fsz = 0
+            with os.fdopen(fd, "wb") as fo:
+                while True:
+                    buf = fi.read(32768)
+                    if not buf:
+                        break
 
-                fsz += len(buf)
-                if fsz > maxsz:
-                    raise Exception("zipbomb defused")
+                    fsz += len(buf)
+                    if fsz > maxsz:
+                        raise Exception("zipbomb defused")
 
-                fo.write(buf)
+                    fo.write(buf)
+        else:
+            wunlink(log, ret, vn.flags if vn else VF_CAREFUL)
+            return None
 
         return ret
 
@@ -422,10 +425,20 @@ def get_cover_from_epub(log: "NamedLogger", abspath: str) -> Optional[IO[bytes]]
             # This might be an EPUB2 file, try the legacy way of specifying covers
             coverimage_path = _get_cover_from_epub2(log, package_root, package_ns)
 
-        # This url is either absolute (in the .epub) or relative to the package document
-        adjusted_cover_path = urljoin(rootfile_path, coverimage_path)
+        if coverimage_path:
+            # This url is either absolute (in the .epub) or relative to the package document
+            adjusted_cover_path = urljoin(rootfile_path, coverimage_path)
 
-        return z.open(adjusted_cover_path)
+            try:
+                return z.open(adjusted_cover_path)
+            except KeyError:
+                log(
+                    "epub: cover specified in package document, but doesn't exist: %s"
+                    % (adjusted_cover_path,)
+                )
+        else:
+            log("epub: no cover found in %s" % (abspath,))
+        return None
 
 
 def _get_cover_from_epub2(
diff --git a/copyparty/th_srv.py b/copyparty/th_srv.py
index 86c3c469..c3ceda02 100644
--- a/copyparty/th_srv.py
+++ b/copyparty/th_srv.py
@@ -378,6 +378,16 @@ class ThumbSrv(object):
 
             if ext in self.args.au_unpk:
                 ap_unpk = au_unpk(self.log, self.args.au_unpk, abspath, vn)
+                if not ap_unpk:
+                    # This file doesn't have a thumbnail
+                    self.worker_cleanup(abspath, ap_unpk, tpath, None, vn)
+                    try:
+                        # create empty thumbnail, otherwise would try again next time
+                        open(tpath, "ab").close()
+                    except OSError:
+                        pass
+                    continue
+
             else:
                 ap_unpk = abspath
 
@@ -451,40 +461,43 @@ class ThumbSrv(object):
                         except:
                             pass
 
-            if abspath != ap_unpk:
-                wunlink(self.log, ap_unpk, vn.flags)
-
             try:
                 atomic_move(self.log, ttpath, tpath, vn.flags)
             except Exception as ex:
                 if not os.path.exists(tpath):
                     t = "failed to move  [%s]  to  [%s]:  %r"
                     self.log(t % (ttpath, tpath, ex), 3)
-                pass
 
-            untemp = []
-            with self.mutex:
-                subs = self.busy[tpath]
-                del self.busy[tpath]
-                self.ram.pop(ttpath, None)
-                untemp = self.untemp.pop(ttpath, None) or []
-
-            for ap in untemp:
-                try:
-                    wunlink(self.log, ap, VF_CAREFUL)
-                except:
-                    pass
-
-            for x in subs:
-                with x:
-                    x.notify_all()
-
-            with self.memcond:
-                self.memcond.notify_all()
+            self.worker_cleanup(abspath, ap_unpk, tpath, ttpath, vn)
 
         with self.mutex:
             self.nthr -= 1
 
+    def worker_cleanup(self, abspath, ap_unpk, tpath, ttpath, vn):
+        if abspath != ap_unpk and ap_unpk:
+            wunlink(self.log, ap_unpk, vn.flags)
+
+        untemp = []
+        with self.mutex:
+            subs = self.busy[tpath]
+            del self.busy[tpath]
+            if ttpath:
+                self.ram.pop(ttpath, None)
+            untemp = self.untemp.pop(ttpath, None) or []
+
+        for ap in untemp:
+            try:
+                wunlink(self.log, ap, VF_CAREFUL)
+            except:
+                pass
+
+        for x in subs:
+            with x:
+                x.notify_all()
+
+        with self.memcond:
+            self.memcond.notify_all()
+
     def fancy_pillow(self, im: "Image.Image", fmt: str, vn: VFS) -> "Image.Image":
         # exif_transpose is expensive (loads full image + unconditional copy)
         res = self.getres(vn, fmt)
@@ -682,7 +695,7 @@ class ThumbSrv(object):
             return
 
         c: Union[str, int] = "90"
-        t = "FFmpeg failed (probably a corrupt video file):\n"
+        t = "FFmpeg failed (probably a corrupt file):\n"
         if (
             (not self.args.th_ff_jpg or time.time() - int(self.args.th_ff_jpg) < 60)
             and cmd[-1].lower().endswith(b".webp")