This is a very common mistake; a lot of people think that some must always be followed by a plural noun. This is not true--it is very easy to use a singular noun after some (as we see here).
And you know what? The funny thing is that we all already know this. Think about the words or phrases someday, someone, some year--they all have something in common.
Can you figure it out?
They all refer to a noun that is unidentified, i.e., a noun that we don't yet know about.
Here's a very common example:
Some guy called you while you were gone.
In this case we use some guy because we don't know who it is.
Hope that helps.