diff --git a/python/tvm/te/operation.py b/python/tvm/te/operation.py
index 3ccab5bfd9c3..b03c6f6f0258 100644
--- a/python/tvm/te/operation.py
+++ b/python/tvm/te/operation.py
@@ -313,6 +313,8 @@ def extern(shape,
     body = fcompute(input_placeholders, output_placeholders)
     if isinstance(body, tvm.tir.PrimExpr):
         body = tvm.tir.Evaluate(body)
+    if not isinstance(body, tvm.tir.Stmt):
+        raise ValueError("Function '{}' should return PrimExpr or Stmt".format(fcompute.__name__))
 
     op = _ffi_api.ExternOp(name, tag, attrs,
                            inputs, input_placeholders,