c++ – Bison: integer out of range

I am trying to make a simple compiler to base the language im trying to make off of. However, while trying to code the simple calculator myself, it pulls an error:

error: integer out of range: '$1'
error: integer out of range: '$2'
error: integer out of range: '$3'

Here is my code:

parser,y

%{
#include<stdio.h>

int regs[26];
int base;

%}

%token<val> NUMBER LETTER PLUS MINU SLAS MULT

%left "+" "-"
%left "*" "/"

%type<val> expr


%%

expr: expr
    | { $$ = $2 }
    | 
    NUMBER PLUS NUMBER
    | { $$ = $1 + $3}
    ;

%%

int main()
{
 return(yyparse());
}

void yyerror(s)
char *s;
{
  fprintf(stderr, "%s\n",s);
}

int yywrap()
{
  return(1);
}

lexer.l

%{

#include <stdio.h>
#include "parser.tab.h"
int c;

%}

%%

[a-zA-Z]*   { return LETTER; }
[0-9]*      { yylval = strtol(yytext, NULL, 10); return NUMBER; }

(\+)        { return PLUS; }
(\*)        { return MULT; }
(\/)        { return SLAS; }
(\-)        { return MINU; }

%%

This code was based off of the IBM example and a few stack overflow posts.

Any ideas? Thanks!