diff --git a/sphinx/util/parallel.py b/sphinx/util/parallel.py
index ce51ac0..64f9bc1 100644
--- a/sphinx/util/parallel.py
+++ b/sphinx/util/parallel.py
@@ -100,6 +100,7 @@ class ParallelTasks(object):
                     raise SphinxParallelError(*result)
                 self._result_funcs.pop(tid)(self._args.pop(tid), result)
                 self._procs[tid].join()
+                self._precvs.pop(tid)
                 self._pworking -= 1
                 break
         else: