错误：无效的第一个参数

Question

我收到以下“无效的第一个参数”错误。但是，con是实际连接并且设置正确。那么这个错误实际上指的是什么？

library(dplyr)
con <- RSQLServer::src_sqlserver("***", database = "***")

myData <- con %>%
  tbl("table") %>%
  group_by( work_dt, campaign, ad_group, matchtype, keyword ) %>%
  select( work_dt, campaign, ad_group, matchtype, keyword, impressions, clicks, cost ) %>%
  filter(site_id %in% c(6932,6946,6948,6949,6951,6952,6953,6954,
                        6955,6964,6978,6979,7061,7260,7272,7329,
                        7791,7794,7850,7858,7983)) %>% 
  filter(work_dt >= as.Date("2014-10-01 00:00:00") & work_dt < as.Date("2014-10-02 00:00:00")) %>%
  summarise(
    sum_impressions = sum(impressions),
    sum_clicks = sum(clicks),
    sum_cost = sum(cost),
  ) %>%
  collect() 

此代码生成：

Error in exists(name, env) : invalid first argument



exists("con")

> exists(con)
Error in exists(con) : invalid first argument
> exists("con")
[1] TRUE