R help - Feb 2015 - get_map in ggplot doesn't allow the exact specification of my box's corners?

Hello!

get_map help says:

location: an address, longitude/latitude pair (in that order), or
left/bottom/right/top bounding box

My code:

library(ggmap)
library(mapproj)

lat_bottom = 52.33  # bottom latitude of Berlin
lat_top    = 52.5   # top latitude of Berlin
lon_left   = 13.0   # left longitude of Berlin
lon_rigth  = 13.95  # right longitude of Berlin

mymap <- get_map(location = c(lon_left,lat_bottom,lon_rigth,lat_top),
source="google")
ggmap(mymap)

Why is it giving me a warning:
Warning: bounding box given to google - spatial extent only approximate.
converting bounding box to center/zoom specification. (experimental)

Does it mean that there is no way for me to create a map with these
exact corners?


Thank you!

-- 
Dimitri Liakhovitski