[R-sig-Geo] Averaging values across grid cells for each layer of a raster brick object

[r@i@1290 m@iii@g oii @im@com]

Hi there,
I am trying to average precipitation values across grid cells of a raster (which is masked to only account for land areas). This raster brick object, called "overlap.sub" has 138 layers (years). It was created as follows:

World_land <- readOGR("ne_110m_land.shp")
newprojection1 <- spTransform(World_land, CRS("+proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0"))overlap <- crop(RCP1pctCO2Median, extent(newprojection1))
overlap.sub <- mask(overlap, newprojection1) #This isolates grid cells on all land areas
The object "overlap.sub" has the following attributes:
class       : RasterBrick 
dimensions  : 62, 127, 7874, 138  (nrow, ncol, ncell, nlayers)
resolution  : 2.8125, 2.789327  (x, y)
extent      : -178.5938, 178.5938, -89.25846, 83.67981  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0 
data source : C:/Users/Travis/AppData/Local/Temp/Rtmp0GALJn/raster/r_tmp_2019-11-14_201408_9732_60407.grd 
names       :    layer.1,    layer.2,    layer.3,    layer.4,    layer.5,    layer.6,    layer.7,    layer.8,    layer.9,   layer.10,   layer.11,   layer.12,   layer.13,   layer.14,   layer.15, ... 
min values  : 0.42964514, 0.43375653, 0.51749371, 0.50838983, 0.45366730, 0.53099146, 0.49757186, 0.45697752, 0.41382199, 0.46082401, 0.45516687, 0.51857087, 0.41005131, 0.45956529, 0.47497867, ... 
max values  :   87.85833,   86.73710,   88.92577,   76.44161,   82.43909,   85.03188,   77.36673,   75.86527,   94.32226,  101.58247,   86.67574,   90.96802,   99.59785,   76.78394,   88.31423, ... 
Now, to obtain the annual averages across the masked grid cells, so that I may plot a time series of the average precipitation across grid cells for Year 1, then for Year 2, then Year 3....all the way to Year 138 (essentially 138 averages):
Averageprec <- colMeans(overlap.sub)
However, this yields this error:
Error in colMeans(overlap.sub) : 
  'x' must be an array of at least two dimensions
I do believe that colMeans is the appropriate function for this, but why am I receiving this error? Unless there is another way to do this? Also, this would only average those grid cells that I masked (i.e. only the land areas), correct? Thanks, and any help/feedback would be greatly appreciated!!
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