ARITHMETIC OPERATION ON TWO TABLES IN MYSQL

I keep getting this error

ERROR 1242 (21000): Subquery returns more than 1 row

After I run the mysql command below

create table CoreIndices14 select Year, ((select SumOfPrecipitation from CoreIndices13)/(select NumberOfDays from CoreIndices12)) as SimpleDailyIntensity from CoreIndices13 group by Year;

I need help. Thanks