Invalid formula - Invalid input expression. - Field name in join query must be fully qualified with

[Centurius_de]

Hi, 

i have a blend of table a and b. 

I understand that if I want to do a calculation on blended data i need to do it on the chart. 
if I now add a calculated field and i try to access the field it always returns null. if I try to add the table name "b" as provided in the blend screen I get the above error message. 

the calculation is super simple. if a.value1 < b.value2 return true. but i am not able to access the value in table b its always null and i don't how i can fix this.