diff --git a/OpenSSL/test/util.py b/OpenSSL/test/util.py
index bc398a1..604a7c2 100644
--- a/OpenSSL/test/util.py
+++ b/OpenSSL/test/util.py
@@ -283,7 +283,7 @@
         """
         if self._temporaryFiles is None:
             self._temporaryFiles = []
-        temp = mktemp(dir=".")
+        temp = b(mktemp(dir="."))
         self._temporaryFiles.append(temp)
         return temp