C# ANTLR4 right-associative warning

Given my simple grammar in the C# build target:

grammar Expr;

prog: stmt+ EOF;
stmt: expr NEWLINE+;

expr:
    expr '^' <assoc=right> expr # Power
|   expr ('*'|'/') expr         # Mult
|   expr ('+'|'-') expr         # Add
|   REAL                        # Real
|   '(' expr ')'                # Paren
;

NEWLINE : '\r\n';
REAL     : [0-9]+'.'[0-9]+ ;

I continue to get a C# compiler warning stating that "Warning 1 rule 'expr' contains an 'assoc' terminal option in an unrecognized location"

Can anyone suggest where I'm supposed to indicate right-associativity for exponentials? This is the way I've seen it done in numerous examples.