diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
index 95c507e..c4a2d9d 100644
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -41,7 +41,7 @@
 }
 
 static void
-invalid_comma_and_underscore()
+invalid_comma_and_underscore(void)
 {
     PyErr_Format(PyExc_ValueError, "Cannot specify both ',' and '_'.");
 }