I have a data frame "customers" build of customer id, month and total purchases that month.
I'm trying to calculate a running slope for a window of 12 months using robust regression.

I have tried the following:

```
Coef <- function(x) {return(rlm(cbind(x)~cbind(1:length(x)))$coefficients[2])}
customer_slope = customers %>% mutate(slope = runner(x=total_purchases,k=12,f=Coef))

```

I get the following error:

> 
> x 'x' is singular: singular fits are not implemented in 'rlm'
> 
> 
> 

If I run a single example, the function returns what I've expected:

```
Coef(c(4,11,7,15,5,14,8,9,14,17,14,13))

```

> 
> cbind(1:length(x))
> 0.6888112
> 
> 
>