diff --git a/paddle/fluid/operators/controlflow/conditional_block_op.cc b/paddle/fluid/operators/controlflow/conditional_block_op.cc
index f64f3f3935955350a340b6fa56e2abea414379d8..4aa400afbf810f5c6c5861a95c01c91b0ff61ab6 100644
--- a/paddle/fluid/operators/controlflow/conditional_block_op.cc
+++ b/paddle/fluid/operators/controlflow/conditional_block_op.cc
@@ -255,6 +255,9 @@ class ConditionalBlockGradOp : public ConditionalOp {
         const auto &input_tensors = input_var->Get<framework::LoDTensorArray>();
         auto *outside_tensors =
             outside_var->GetMutable<framework::LoDTensorArray>();
+        if (outside_tensors->size() == 0U) {
+          outside_tensors->resize(input_tensors.size());
+        }
         PADDLE_ENFORCE_EQ(input_tensors.size(),
                           outside_tensors->size(),
                           platform::errors::InvalidArgument(