diff --git a/OpenSSL/util.h b/OpenSSL/util.h
index 73accbf..f9e3a77 100644
--- a/OpenSSL/util.h
+++ b/OpenSSL/util.h
@@ -131,4 +131,10 @@
 #define PY_SSIZE_T_MIN INT_MIN
 #endif
 
+#if !defined(PyNumber_ToBase)
+extern PyObject* PyOpenSSL_LongToHex(PyObject *o);
+#else
+#define PyOpenSSL_LongToHex(o) PyNumber_ToBase(o, 16)
+#endif
+
 #endif