diff --git a/psycopg/bytes_format.c b/psycopg/bytes_format.c
index 991f7605..cd06b1cc 100644
--- a/psycopg/bytes_format.c
+++ b/psycopg/bytes_format.c
@@ -111,7 +111,7 @@ Bytes_Format(PyObject *format, PyObject *args)
     PyObject *result;
     PyObject *dict = NULL;
     if (format == NULL || !Bytes_Check(format) || args == NULL) {
-        PyErr_BadInternalCall();
+        PyErr_SetString(PyExc_SystemError, "bad argument to internal function");
         return NULL;
     }
     fmt = Bytes_AS_STRING(format);