How to apply ISERROR in an Array Formula

Attached is a simple example of what I’m trying to achieve. There are differing money amounts in column C and the month those amounts are due to be paid in column B. Column E contains an array formula based upon a lookup value in cell G3 (month due for payment). The formula works fine but of course will report #NUM errors when it runs out of values to find from G3. In this example, I wanted J3 to return the total amount due in Jan but of course it also returns a #NUM error as it references the total error in E18. I have tried ISERROR but cannot get it to work – am I on the right track or is there a simpler alternative?