diff --git a/tests/wpt/web-platform-tests/tools/sslutils/openssl.py b/tests/wpt/web-platform-tests/tools/sslutils/openssl.py
index 2bbc0fa7545..754bc946c7e 100644
--- a/tests/wpt/web-platform-tests/tools/sslutils/openssl.py
+++ b/tests/wpt/web-platform-tests/tools/sslutils/openssl.py
@@ -252,7 +252,10 @@ class OpenSSLEnvironment(object):
         with open(path("index.txt"), "w"):
             pass
         with open(path("serial"), "w") as f:
-            f.write(str(random.randint(0, 1000000)))
+            serial = str(random.randint(0, 1000000))
+            if len(serial) % 2:
+                serial = "0" + serial
+            f.write(serial)
 
         self.path = path