diff --git a/gio/gmemoryoutputstream.c b/gio/gmemoryoutputstream.c
index 82747a3ab..6611eada8 100644
--- a/gio/gmemoryoutputstream.c
+++ b/gio/gmemoryoutputstream.c
@@ -828,6 +828,8 @@ g_memory_output_stream_truncate (GSeekable     *seekable,
   if (!array_resize (ostream, offset, FALSE, error))
     return FALSE;
 
+  ostream->priv->valid_len = offset;
+
   return TRUE;
 }
 
diff --git a/gio/tests/memory-output-stream.c b/gio/tests/memory-output-stream.c
index a8b389a7d..b9be0df6b 100644
--- a/gio/tests/memory-output-stream.c
+++ b/gio/tests/memory-output-stream.c
@@ -50,6 +50,11 @@ test_truncate (void)
       g_assert_no_error (error);
     }
 
+  g_test_bug ("720080");
+
+  g_seekable_truncate (G_SEEKABLE (mo), 8192, NULL, &error);
+  g_assert_cmpint (g_memory_output_stream_get_data_size (G_MEMORY_OUTPUT_STREAM (mo)), ==, 8192);
+
   g_object_unref (o);
   g_object_unref (mo);
 }