diff --git a/libvips/create/buildlut.c b/libvips/create/buildlut.c
index 48bfa0f7..16ab774d 100644
--- a/libvips/create/buildlut.c
+++ b/libvips/create/buildlut.c
@@ -277,7 +277,8 @@ vips_buildlut_init( VipsBuildlut *lut )
  * @...: %NULL-terminated list of optional named arguments
  *
  * This operation builds a lookup table from a set of points. Intermediate
- * values are generated by piecewise linear interpolation. 
+ * values are generated by piecewise linear interpolation. The lookup table is
+ * always of type %gdouble, use vips_cast() to change it to the type you need.
  *
  * For example, consider this 2 x 2 matrix of (x, y) coordinates:
  *