scRNA.sub =subset(scRNAnew, downsample=30) ，argument "downsample" didn't work,what ‘s wrong?

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scRNA.sub =subset(scRNAnew, downsample=30) ，argument "downsample" didn't work,what ‘s wrong?

Idents(scRNAnew) = "celltype"
scRNA.sub =subset(scRNAnew, downsample=30) # downsample函数实现分层抽样 每个细胞类型抽200个
scRNA.sub
An object of class Seurat
29984 features across 300 samples within 2 assays
Active assay: RNA (27984 features, 0 variable features)
1 other assay present: integrated
3 dimensional reductions calculated: pca, umap, tsne
table(scRNA.sub$celltype)

Fibroblasts NK_cells T_cells Epithelial_cells Macrophages
89 88 23 31 25
Monocytes Plasma_cells SMC Endothelial_cells B_cells
13 10 8 11 2