diff --git a/libgimp/gimpbrushselectbutton.c b/libgimp/gimpbrushselectbutton.c
index 759348244d..e7761076cf 100644
--- a/libgimp/gimpbrushselectbutton.c
+++ b/libgimp/gimpbrushselectbutton.c
@@ -406,6 +406,7 @@ gimp_brush_select_button_get_brush_bitmap (GimpBrushSelectButton *self)
   GimpBrush                    *brush;
   gint                          mask_bpp;
   GBytes                       *mask_data;
+  gsize                         mask_size;
   gint                          color_bpp;
   GBytes                       *color_data;
   _PreviewBitmap                result;
@@ -422,7 +423,7 @@ gimp_brush_select_button_get_brush_bitmap (GimpBrushSelectButton *self)
                          &color_bpp,
                          &color_data);
 
-  result.mask_data = g_bytes_unref_to_data (mask_data, NULL);
+  result.mask_data = g_bytes_unref_to_data (mask_data, &mask_size);
   /* Discard any color data, bitmap is B&W i.e. i.e. depth one i.e. a mask */
   g_bytes_unref (color_data);
 
diff --git a/libgimp/gimpdrawable.c b/libgimp/gimpdrawable.c
index 5a21e955bd..fba3f0469c 100644
--- a/libgimp/gimpdrawable.c
+++ b/libgimp/gimpdrawable.c
@@ -133,6 +133,7 @@ gimp_drawable_get_thumbnail (GimpDrawable           *drawable,
 {
   gint       thumb_width, thumb_height, thumb_bpp;
   GBytes    *data;
+  gsize      data_size;
   GdkPixbuf *pixbuf = NULL;
 
   g_return_val_if_fail (width  > 0 && width  <= 1024, NULL);
@@ -146,7 +147,7 @@ gimp_drawable_get_thumbnail (GimpDrawable           *drawable,
                                            &thumb_bpp);
 
   if (data)
-    pixbuf = _gimp_pixbuf_from_data (g_bytes_unref_to_data (data, NULL),
+    pixbuf = _gimp_pixbuf_from_data (g_bytes_unref_to_data (data, &data_size),
                                      thumb_width, thumb_height, thumb_bpp,
                                      alpha);
 
@@ -238,6 +239,7 @@ gimp_drawable_get_sub_thumbnail (GimpDrawable           *drawable,
   gint       thumb_height = dest_height;
   gint       thumb_bpp;
   GBytes    *data;
+  gsize      data_size;
   GdkPixbuf *pixbuf = NULL;
 
   g_return_val_if_fail (src_x >= 0, NULL);
@@ -256,7 +258,7 @@ gimp_drawable_get_sub_thumbnail (GimpDrawable           *drawable,
                                                &thumb_bpp);
 
   if (data)
-    pixbuf = _gimp_pixbuf_from_data (g_bytes_unref_to_data (data, NULL),
+    pixbuf = _gimp_pixbuf_from_data (g_bytes_unref_to_data (data, &data_size),
                                      thumb_width, thumb_height, thumb_bpp,
                                      alpha);
   g_bytes_unref (data);