diff --git a/src/tensorrt/impl/tensorrt_runtime_opr.cpp b/src/tensorrt/impl/tensorrt_runtime_opr.cpp
index f1864ee9..ac6d7dfa 100644
--- a/src/tensorrt/impl/tensorrt_runtime_opr.cpp
+++ b/src/tensorrt/impl/tensorrt_runtime_opr.cpp
@@ -194,21 +194,25 @@ void TensorRTRuntimeOpr::init_output_dtype() {
         idx++;
     }
 
-    for (size_t i = 0; i < output().size(); ++i) {
+    size_t out = 0;
+    for (; out < output().size() - 1; ++out) {
         dt_trt = get_dtype_from_trt(m_engine->getBindingDataType(idx));
         mgb_assert(
                 dt_trt.valid(),
                 "output dtype checking failed: invalid dtype returned.");
         if (dt_trt.enumv() == DTypeEnum::QuantizedS8) {
             mgb_assert(
-                    output(i)->dtype().valid(),
+                    output(out)->dtype().valid(),
                     "user should specify scale of output tensor of "
                     "TensorRTRuntimeOpr.");
         }
-        if (!output(i)->dtype().valid())
-            output(i)->dtype(dt_trt);
+        if (!output(out)->dtype().valid())
+            output(out)->dtype(dt_trt);
         idx++;
     }
+    //! workspace
+    if (!output(out)->dtype().valid())
+        output(out)->dtype(dtype::Byte());
 }
 
 SymbolVarArray TensorRTRuntimeOpr::make(