# EvalAdd does not add in some cases

**URL:** <https://openfhe.discourse.group/t/evaladd-does-not-add-in-some-cases/517>\
**Category:** Library Questions\
**Created:** [May 2, 2023, 12:17pm UTC](https://openfhe.discourse.group/t/evaladd-does-not-add-in-some-cases/517 "2023-05-02T12:17:15Z")\
**Posts on this page:** 1\
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**Author:** ![ypolyakov](https://yyz1.discourse-cdn.com/flex031/user_avatar/openfhe.discourse.group/ypolyakov/32/47_2.png) [@ypolyakov](https://openfhe.discourse.group/u/ypolyakov)\
**Post date:** [May 5, 2023, 3:41pm UTC](https://openfhe.discourse.group/t/evaladd-does-not-add-in-some-cases/517/7 "2023-05-05T15:41:24Z")

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Hi @lujoho

Please keep in mind that `scaleModSize` and `firstModSize` are two related parameters. `firstModSize` is used for decryption. If the multiplicative depth is set exactly to the depth of circuit, then at the time of decryption you will be working with a single modulus of `firsModSize`. Therefore, your message cannot be larger (in bits) than `firstModSize` - `scaleModSize` (roughly). When you increase `scaleModSize` (closer to `firstModulusSize`), you decrease the room for the message. Most likely, you are getting into this issue (it is a question of using CKKS correctly rather than a bug in OpenFHE).

There is an easy way to confirm this. Just increase the multiplicative depth by 1 and see if the issue goes away. If you do this, the message “budget” in bits will increase to `firstModSize`, i.e., `scaleModSize` + `firstModSize` - `scaleModSize`.

Everything I described here is expected by CKKS design.

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