email V2 attachment value cannot be null

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Posted on by MP-30041046-0

We have multiple flows sending mails with attachments. All of them are build the same way, getting the attachments from a sharepoint list, adding them to a variable and then sending the mails. Two of the flows suddenly stopped working and now always throw an error "Value cannot be null. Parameter name: s at System.COnvert.FromBase64String(string s)." All the other flows with the same setup still work.

Anyone has the same problem and knows a solution for this?

Error:

adding content to the variable:

Attachment in the mail:

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Building flows

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rzaneti 4,476 Super User 2026 Season 1 on at

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Hi @MP-30041046-0,

Could you please check the value of the Attachment Mail variable after the append operation? You can simply add a Compose right after the append to print the variable's value.

It is possible that your need to enclose the dynamic contents with double quotes to represent them as strings inside the object. If you don't enclose it, then Power Automate will interpret this object as a single big string, leading to this issue in the Send email action (which expects an array of objects, and will receive an array of strings instead).

Let me know if it works for you or if you need any additional help!

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MP-30041046-0 2 on at

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Hi @rzaneti,

thanks for the answer. I tried as you suggested but the string is enclosed in the compose:

I also tried encosing the dynamic content with the same result. It is also strange as it worked for month without any problems but just suddenly stopped working. And other workflows build the same way still work...

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rzaneti 4,476 Super User 2026 Season 1 on at

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Hi @MP-30041046-0,

I think I got it. You need to capitalize the C in ContentBytes.

This seems to be a recent change in the Outlook connector, where the contentBytes (lower-case) was updated to ContentBytes (upper-case). I found it here: https://learn.microsoft.com/en-ie/answers/questions/2238771/what-is-causing-this-error-value-cannot-be-null-rn

Let me know if it works for you or if you need any additional help!

✅ If this solved your issue, please mark it as Accepted Answer.
👍 If it helped, feel free to give it a like!

🌐 Explore more Power Platform content on my Website or on my ▶️ YouTube
💼 Find me on LinkedIn

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