Experience validation - previous employment was with unregistered startup

Recently, I applied for a job in a company wherein I mentioned that I had worked for a company for six months during the beginning of my career - which is true. However, the company was a startup and it was unregistered. We worked under a guy who used to pass on freelancing work to us.
I mentioned to the HR that I will only be able to provide them with the experience certificate and no proof of receipt of salary. Will this create problem during background check? As I have already mentioned to them that it was a startup but have not mentioned that it was un-registered as I also got to know about the same.

This shouldn't affect the BGV. Yet to be on a safer side in the future, just mention that you were freelancing.

Hello Rahul,
Thank you for your time on this response,
Generally how the system works? As, I know background checks can be lethal to the carrier of an individual if there is any problem, in my case I did not lie to be honest.. I am just skeptical if the company would term it as a forgery as I am Not sure if this would be taken into consideration legitimately as the company was not registered, given the fact that there are many startups that work this way (unregistered - provided the fact that the cost of privatization registration is high, startups generally do not do it to save money initially) looking for some more insight. I already joined the organization now and have mentioned this fact clearly twice in an email looping project managers and HR personnel, but I did not mention about the unregistered thing as I was also unaware.
Regards,

Company would term it as forgery if would have lied about your experience and created fake certificates to substantiate the same. To be more precise, with the advent of technology, the world indeed has become very transparent and small; and this makes BG eazy for the agencies or employers.

Hello Rahul,
So even after me informing the HR's (written medium) prior to my employment it will be considered a forgery? I legitimately received the experience letter from the owner of the company..
In my case..
1. I already informed them that it was a startup and I cannot give them any payment proof as mode of payment was in cash.
2. I Informed them that I can only give them the experience certificate from the employer and nothing much.
3. I looped in project managers in this email prior to even receiving the offer letter.
4. I clearly stated that it was a small tech startup and there is no employee code that I had during my tenure.
5. I clearly mentioned that the startup was unsuccessful and has now been closed.
Note: All this information was presented even before receiving the offer letter.
There was no forgery of document.
Please let me know your thoughts for this.

Additionally,
6. The work experience was of only 5 months.
7. I Asked them that if they cannot consider this to be valid experience they must process my application with the exclusion of this work experience.
Regards,