The boundary_line is empty when I used with st.dd.identify_boundary

[yang050036]

The boundary_line is empty when I used with st.dd.identify_boundary

bdl_img = st.dd.identify_boundary(adata=adata, spatial_key="spatial", cluster_key="leiden_0.2", bin_size=1, source_id=['1', '3'], target_id=['2', '7'])
st.pl.space(adata, color=['boundary_line'], pointsize=0.2, cmap = "Reds")

What should I do in the next?