diff abundance after aggTax

[JHBI115]

Hi ,
I'm not a statistician and not very pro-efficient in R.
I'm trying to run fitFeatureModel on the MRexperiment object that was output by aggTax function . And I get this ERROR ;
Error in lmFit(log(mat), mod, weights = weights) :
row dimension of design doesn't match column dimension of data object

My intial obj :

obj_Treat
MRexperiment (storageMode: environment)
assayData: 2549 features, 48 samples
element names: counts
protocolData: none
phenoData
sampleNames: F110-2 F110-d ... F142-d (48 total)
varLabels: patientID preterm_del ... treatment2 (12 total)
varMetadata: labelDescription
featureData
featureNames: 36ba457bd2f0549ccde2199095e7aa18
b3212620482d3313452d1cce8bfb2f73 ...
d1b3d9b8720fb1e710a3d20a348b8388 (2549 total)
fvarLabels: OTUID Taxon ... Species (9 total)
fvarMetadata: labelDescription
experimentData: use 'experimentData(object)'
Annotation:

Class = aggTax(obj_Treat, lvl = "Class", out = "MRexperiment", returnFullHierarchy=FALSE)
Class
MRexperiment (storageMode: environment)
assayData: 41 features, 48 samples
element names: counts
protocolData: none
phenoData
sampleNames: F110-2 F110-d ... F142-d (48 total)
varLabels: patientID preterm_del ... treatment2 (12 total)
varMetadata: labelDescription
featureData
featureNames: [Chloracidobacteria] [Saprospirae] ... ZB2 (41 total)
fvarLabels: Taxa
fvarMetadata: labelDescription
experimentData: use 'experimentData(object)'
Annotation:

Class = cumNorm(Class, p = 0.5)
pd = pData(Class)
mod <- model.matrix(~1 + treatment, data = pd)
Class_flouride_vs_placebo = fitFeatureModel(Class, mod)
Error in lmFit(log(mat), mod, weights = weights) :
row dimension of design doesn't match column dimension of data object

dim(mod)
[1] 48 2

What am I doing wrong ?

Thank you so much !
Cori