NA values in similarity matrix R

Question

I am working on a Book recommender system using R. Below is the dataset

> str(book_ratings)
'data.frame':   492134 obs. of  3 variables:
 $ User.ID    : int  276725 276726 276727 276729 276729 ...
 $ ISBN       : int  34545104 15506122 44652080 52165615 52179502 ...
 $ Book.Rating: int  0 5 0 3 6 0 8 6 7 10 ...

When i tried to do a similarity matrix, it shows NA except the diagonals. Below is the code:

book_matrix <- as(book_ratings, "realRatingMatrix")
similarity_users <- similarity(book_matrix[1:4, ], method = "cosine",
                               which = "User.ID")

as.matrix(similarity_users)

   2  7  8  9
2  0 NA NA NA
7 NA  0 NA NA
8 NA NA  0 NA
9 NA NA NA  0

Instead of NA I wanted to see the correlation between the first 4 users. Please correct my understanding if it is wrong. This is the first time I am working on a recommender system.