diff --git a/tests/fpgadataflow/test_npy2hls.py b/tests/fpgadataflow/test_npy2hls.py
index 884c16cc3dcae72352b1f14d20c508f81d2f425c..f2956a85a5957efa04a08cc1a226d283a2313086 100644
--- a/tests/fpgadataflow/test_npy2hls.py
+++ b/tests/fpgadataflow/test_npy2hls.py
@@ -32,6 +32,7 @@ def make_npy2apintstream_testcase(ndarray, dtype):
     npy_type = npyt_to_ct[str(ndarray.dtype)]
     shape_cpp_str = str(shape).replace("(", "{").replace(")", "}")
     test_app_string = []
+    test_app_string += ["#include <cstddef>"]
     test_app_string += ['#include "ap_int.h"']
     test_app_string += ['#include "stdint.h"']
     test_app_string += ['#include "hls_stream.h"']