adding contours to a projected map

Hello,

I'm having a problem adding contours to a projected map. I was able to do it
before I specified a projection. This is the code:

Sum.80.83.li <-

interp(SummerO18.80.84.df$longitude,SummerO18.80.84.df$latitude,
+ SummerO18.80.84.df$wt18o)

eurM <- map(database = "world", interior = FALSE, xlim = c(-10,50),

+ ylim = c(35,60), plot = FALSE)

map("world", interior = FALSE, xlim = c(-10,50), ylim = c(35,60),

+ projection = "bonne", par = 47, col = "slateblue1")

map.grid(eurM, labels = FALSE, col = "lightgray", lty = 2, nx = 2, ny = 2)
contour(Sum.80.83.li, add = TRUE)

Everything was coming out nicely until I tried to add the contours. No
contours printed on the map. Any ideas on what is going wrong?

Thanks,
Jeanne