Sharing SharePoint Online Site with External Users

Scenario:
I have subscription of SharePoint Online and have created few private sites in it. I wanted to share one of the private site collection with the external users and keep on getting the following error message:

Sorry, you are not allowed to share this with external users.

However when I try to share existing sites (default sites) with external users, it gets shared without any problem.

Solution:
For the new site collections you have to enable following option:

1. Go to the SharePoint admin center and  select the Site Collection.
2. Select Sharing from the tool bar. 
3. Change the sharing settings to Allow