Beginner's query - segmentation fault

I am dealing with a huge matrix in R (20 columns, 54000 rows) and have
lots of missing values within the dataset which are currently displayed as
the value "-999.00" I am trying to create a new matrix (or change the
existing one) to display these values as "NA" so that I can then perform
the necessary analysis on the columns within the matrix.

The matrix name is temp and the column names are t1 to t20 inclusive.

I have tried the following command:

temp$t1[temp$t1 == -999.00] <- NA

and it returns a segmentation fault, can someone tell me what I am doing
wrong?

Thanks
Laura