diff --git a/tests/libtest/lib650.c b/tests/libtest/lib650.c
index ac56c4f35b..04b288725f 100644
--- a/tests/libtest/lib650.c
+++ b/tests/libtest/lib650.c
@@ -92,7 +92,7 @@ static CURLcode test_lib650(const char *URL)
   formrc = curl_formadd(&formpost,
                         &lastptr,
                         CURLFORM_PTRNAME, testname,
-                        CURLFORM_NAMELENGTH, (long)(sizeof(testname) - 2),
+                        CURLFORM_NAMELENGTH, (long)CURL_CSTRLEN(testname) - 1,
                         CURLFORM_ARRAY, formarray,
                         CURLFORM_FILENAME, "remotefile.txt",
                         CURLFORM_END);