Receating the R package tigris example for Alaska. Single digit state codes and double digit county codes have no leading zeros and won't merge.

AK_example.R

library(tigris)

dfw <- tracts(state = '02', county = '020', detailed = FALSE)

library(acs)
library(leaflet)

api.key.install("API_KEY" )

income_data <- acs.fetch(endyear = 2012, 
                         geography = geo.make(state = "AK", 
                                              county = 020, 
                                              tract = "*"), 
                         variable = "B19013_001")

#Resolve acs.fetch triming zeros. 
#income_data@geography$state[nchar(income_data@geography$state) == 1]  <- paste0(0, income_data@geography$state)
#income_data@geography$county[nchar(income_data@geography$county) == 2]  <- paste0(0, income_data@geography$county)

income_df <- data.frame(paste0(as.character(income_data@geography$state), 
                               as.character(income_data@geography$county), 
                               income_data@geography$tract), 
                               income_data@estimate)

colnames(income_df) <- c("GEOID", "hhincome")

dfw_merged <- geo_join(dfw, income_df, "GEOID", "GEOID")

pal <- colorQuantile("Greens", NULL, n = 6)

popup <- paste0("Median household income: ", as.character(dfw_merged$hhincome))

leaflet() %>%
  addProviderTiles("CartoDB.Positron") %>%
  addPolygons(data = dfw_merged, 
              fillColor = ~pal(dfw_merged$hhincome), 
              fillOpacity = 0.7, 
              weight = 0.2, 
              popup = popup) %>%
  addLegend(pal = pal, 
            values = dfw_merged$hhincome, 
            position = "bottomright", 
            title = "Income in ANC")

dfw@data$GEOID[1]
income_df$GEOID[1] #leading zeros for state and county are trimed in acs.fetch