I try to run a tutorial on this line

exprs(my.rma) <- (exprs(my.rma)[present >= 4, ])

I got an error :

Error in .validate_assayDataElementReplace(obj, value) : 
  object and replacement value have different dimensions

I checked both side of <- are class "matrix" "array".

I can run this line :

table(present >= 4) 
output:    FALSE  TRUE 
                  27820 26855

Thank you