R 'differing number of rows' error using OpenWeather API - r

Question

For an assignment I'm trying to fetch weather forecast data from the OpenWeather API. My code returns this error:
Error in data.frame(city = city, weather = weather, visibility = visibility, :
arguments imply differing number of rows: 40, 1, 0, 12
I clicked to re-run with RStudio's debug and got:
Not sure what's causing this error. Can anyone help me find the problem in my code? Very grateful for any help.
# To create empty vectors to hold data temporarily
city <- c()
weather <- c()
visibility <- c()
temp <- c()
temp_min <- c()
temp_max <- c()
pressure <- c()
humidity <- c()
wind_speed <- c()
wind_deg <- c()
forecast_datetime <- c()
season <- c()
# To get 5-day forecast data for select cities
get_weather_forecast_by_cities <- function(city_names){
df <- data.frame()
for (city_name in city_names){
url_get='https://api.openweathermap.org/data/2.5/forecast'
weather_api_key <- "[*my API key*]"
forecast_query <- list(q = city_name, appid = weather_api_key, units="metric")
response <- GET(url_get, query=forecast_query)
json_list <-content(response, as="parsed")
results <- json_list$list
for(result in results) {
# Get weather data and append them to vectors
city <- c(city, city_name)
}
# Combine into a DF
weather <- c(weather, result$weather[[1]]$main)
visibility <- c(visibility, result$visibility)
temp <- c(temp, result$main$temp)
temp_min <- c(temp_min, result$main$temp_min)
temp_max <- c(temp_max, result$main$temp_max)
pressure <- c(pressure, result$main$pressure)
humidity <- c(humidity, result$main$humidity)
wind_speed <- c(wind_speed, result$wind$speed)
wind_deg <- c(wind_deg, result$wind$deg)
forecast_datetime <- c(forecast_datetime, result$dt_text)
months <- as.numeric(format(as.Date(forecast_datetime), '%m'))
index <- setNames(rep(c('winter', 'spring', 'summer', 'fall'), each=3), c(12,1:11))
season <- unname(index[as.character(months)])
weather_df <- data.frame(city=city, weather=weather, visibility=visibility, temp=temp, temp_min=temp_min, temp_max=temp_max, pressure=pressure, humidity=humidity, wind_deg=wind_deg, forecast_datetime=forecast_datetime, months=months, index=index, season=season)
}
return(df)
}
cities <- c("Seoul", "New York City", "Paris", "London", "Taiyuan")
cities_weather_df <- get_weather_forecast_by_cities(cities)