diff --git a/numpy/f2py/tests/test_callback.py b/numpy/f2py/tests/test_callback.py
index 335b78413db9..018cea4fd5e3 100644
--- a/numpy/f2py/tests/test_callback.py
+++ b/numpy/f2py/tests/test_callback.py
@@ -116,7 +116,7 @@ def callback(cu, lencu):
 
         f = getattr(self.module, "string_callback_array")
         for cu in [cu1, cu2, cu3]:
-            res = f(callback, cu, len(cu))
+            res = f(callback, cu, cu.size)
             assert res == 0
 
     def test_threadsafety(self):