Error In UseMethod("doResolveFormula")

I'm using the following code to put three different types of markers for each level of a categorical variable RSPM.

leaflet(df) %>% addTiles() %>%
addCircleMarkers(~Long, ~Lat, popup = ~as.character(RSPM),
radius = ~if(RSPM=="Critical"){
  10
}else if (RSPM=="High"){
  6
} else if (RSPM=="Moderate")
  2,
color = ~pal(type),
stroke = FALSE, fillOpacity =  0.5
)

The following is the error message:

Error in UseMethod("doResolveFormula") : 
  no applicable method for 'doResolveFormula' applied to an object of class "function"

Don't know what this error message is referring to, and how to fix it. This is using leaflet.