diff --git a/C/dbase.c b/C/dbase.c
index 7ad9669a2..65b1e0d1b 100644
--- a/C/dbase.c
+++ b/C/dbase.c
@@ -4631,6 +4631,7 @@ p_instance(void)
 	if (IsVarTerm(t2)) {
 	  Yap_unify(ARG2, (t2 = Yap_MkNewApplTerm(f,arity)));
 	} else if (!IsApplTerm(t2) || FunctorOfTerm(t2) != f) {
+	  UNLOCK(ap->PELock);
 	  return FALSE;
 	}
 	ptr = RepAppl(t2)+1;