I am trying to convert a rather large (29k rows and 116k columns) SingleCellExperiment object to a AnnData object on disk using the writeH5AD function from the zellkonverter package. The matrix I want to use as the primary assay is a LowRankMatrix class and by default zellkonverter doesn't know how to convert this to a dgCMatrix so instead converts it to a base matrix class which is too large to fit in memory. The LowRankMatrix is actually the "reconstructed" assay generated by the correctExperiments function from the batchelor package. When I try to manually coerce the LowRankMatrix to a dgCMatrix I receive the following error:

```
library(Matrix)
library(SingleCellExperiment)
sce <- readRDS("SCE.rds")
mat <- assay(sce, "reconstructed")
mat <- as(mat, "dgCMatrix")
Error in rbind(x, y, z) : negative extents to matrix
```

The operation is however successful if I perform it on a smaller subset of the matrix:

```
mat <- mat[1:100, 1:100]
mat <- as(mat, "dgCMatrix")
```

I don't strictly need a H5AD file, I could get by with an MTX file, however the writeMM function from the Matrix package doesn't know how to handle a LowRankMatrix either.

I've opened a GitHub issue here as a reminder to fix this https://github.com/theislab/zellkonverter/issues/32.