Getting API error "is invalid" when trying to update order tags

[DanAtBonify]

I have a customer with a prevalent and intermittent issue of getting an error when trying to update order tags.

The error message returned from the API:

is invalid

The strange thing is that when I then do another attempt to update the order tags the API call will work, so the "is invalid" error seems to be intermittent, and I'm not sure what's causing it.

I'm hoping that someone from Shopify can clarify what kinds of things can cause that error, because it is showing up often, even though retrying the API call will work.

Thanks!

P.S. Some examples of the tags attempted to be added (updated): 

Time_Selected: 02:00PM-04:00PM, Status: Unfulfilled, Carrier: Postmates, Postmates Unfulfilled

[npabisz]

In my case this was caused by changing letters to lower case in existing tags. Eg. I had existing tag "Sub", but did request with "sub".

[lam_af]

did you find the cause? doesn’t seem to be related to CaSe for us