diff --git a/gio/gconverter.c b/gio/gconverter.c
index 9dacaf03e..57ee1b904 100644
--- a/gio/gconverter.c
+++ b/gio/gconverter.c
@@ -57,12 +57,14 @@ g_converter_default_init (GConverterInterface *iface)
  * @inbuf: (array length=inbuf_size) (element-type guint8): the buffer
  *         containing the data to convert.
  * @inbuf_size: the number of bytes in @inbuf
- * @outbuf: (element-type guint8) (array length=outbuf_size): a buffer to write
- *    converted data in.
+ * @outbuf: (element-type guint8) (array length=outbuf_size) (not nullable): a
+ *    buffer to write converted data in.
  * @outbuf_size: the number of bytes in @outbuf, must be at least one
  * @flags: a #GConverterFlags controlling the conversion details
- * @bytes_read: (out): will be set to the number of bytes read from @inbuf on success
- * @bytes_written: (out): will be set to the number of bytes written to @outbuf on success
+ * @bytes_read: (out) (not nullable): will be set to the number of bytes read
+ *    from @inbuf on success
+ * @bytes_written: (out) (not nullable): will be set to the number of bytes
+ *    written to @outbuf on success
  * @error: location to store the error occurring, or %NULL to ignore
  *
  * This is the main operation used when converting data. It is to be called
@@ -166,7 +168,12 @@ g_converter_convert (GConverter *converter,
   GConverterIface *iface;
 
   g_return_val_if_fail (G_IS_CONVERTER (converter), G_CONVERTER_ERROR);
+  g_return_val_if_fail (inbuf != NULL || inbuf_size == 0, G_CONVERTER_ERROR);
+  g_return_val_if_fail (outbuf != NULL, G_CONVERTER_ERROR);
   g_return_val_if_fail (outbuf_size > 0, G_CONVERTER_ERROR);
+  g_return_val_if_fail (bytes_read != NULL, G_CONVERTER_ERROR);
+  g_return_val_if_fail (bytes_written != NULL, G_CONVERTER_ERROR);
+  g_return_val_if_fail (error == NULL || *error == NULL, G_CONVERTER_ERROR);
 
   *bytes_read = 0;
   *bytes_written = 0;