Pairwise correlation

Hiya

I have a problem regarding the pairwise correlation. Following is the output format:


Var1 Var2 Var3 Var4
VAR1 1
VAR2 0.98* 1 0.9* 0.9
VAR3 0.09 0.02 1 0.34
VAR4 . . . 1


My query here is why there is a "." in VAR4?


Thank you