[R-sig-Geo] Calculating return level with return period

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

Hi there,
I am trying to compute return levels for a certain return period (20 years) for my extreme precipitation. Using the block maxima approach, I made the following object:
    Gcomb <- c(Gmax[1], Gmax1[1], Gmax2[1], Gmax3[1], Gmax4[1], Gmax5[1], Gmax6[1], Gmax7[1], Gmax8[1],     Gmax[2], Gmax1[2], Gmax2[2], Gmax3[2], Gmax4[2], Gmax5[2], Gmax6[2], Gmax7[2], Gmax8[2], Gmax[3],     Gmax1[3], Gmax2[3], Gmax3[3], Gmax4[3], Gmax5[3], Gmax6[3], Gmax7[3], Gmax8[3], Gmax[4], Gmax1[4],     Gmax2[4], Gmax3[4], Gmax4[4], Gmax5[4], Gmax6[4], Gmax7[4], Gmax8[4], Gmax[5], Gmax1[5], Gmax2[5],     Gmax3[5], Gmax4[5], Gmax5[5], Gmax6[5], Gmax7[5], Gmax8[5], Gmax[6], Gmax1[6], Gmax2[6], Gmax3[6],     Gmax4[6], Gmax5[6], Gmax6[6], Gmax7[6], Gmax8[6], Gmax[7], Gmax1[7], Gmax2[7], Gmax3[7], Gmax4[7],     Gmax5[7], Gmax6[7], Gmax7[7], Gmax8[7])
Now, when I try to compute the return level (a 20-year return level):
    return.level.fevd(Gcomb, return.period = 20)
I end up with the following error:
    Error: $ operator is invalid for atomic vectors
Why is this error showing up? Is there any way to avoid this?
Thanks, and any help would be greatly appreciated!
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