diff --git a/libvips/include/vips/private.h b/libvips/include/vips/private.h
index da39bd55..62489791 100644
--- a/libvips/include/vips/private.h
+++ b/libvips/include/vips/private.h
@@ -170,7 +170,7 @@ typedef int (*im_region_fill_fn)( struct _REGION *, void * );
 int im_region_fill( struct _REGION *reg, Rect *r, im_region_fill_fn fn, void *a );
 void im_region_print( struct _REGION *region );
 
-int im_prepare_many( REGION **reg, Rect *r );
+int im_prepare_many( struct _REGION **reg, Rect *r );
 
 #ifdef __cplusplus
 }