ORA-06502: PL/SQL: numeric or value error: character to number conversion error

Im a bit stumped when getting this error with the following procedure: Any ideas?

ORA-06502: PL/SQL: numeric or value error: character to number conversion error
ORA-06512: at "SYS.P_REPLACE_SCHEMA", line 10
ORA-06512: at line 1

procedure P_REPLACE_SCHEMA
as
UserCount number(1);
UserName dba_users.username%TYPE;
cid integer;
userCheckSql varchar2(2000);
BEGIN
UserName := upper('girs');
userCheckSql := 'SELECT NVL(count(1),0) FROM dba_users WHERE username = :s' ;
UserCount := 'EXECUTE IMMEDIATE userCheckSql USING UserName';
IF (UserCount = 1)
THEN
cid := dbms_sql.open_cursor;
dbms_sql.parse(cid, 'DROP user girs cascade' , dbms_sql.native);
dbms_sql.close_cursor(cid);
dbms_output.put_line('User girs dropped');
ELSE
dbms_output.put_line('No user girs to drop');
END IF;
END;

You are trying to set UserCount, a number, to the string "EXECUTE IMMEDIATE userCheckSql USING UserName", which will not convert to a number for obvious reasons