If you are using SSMS V17.4 as I am

You may encounter a weird error. This can be repeated as follows:

Open a new SSMS window, and connect to any SQL Server instance (I tested on connections to SQL Server 2012/2014/2016/2017), and then execute the following

set showplan_all on;

and then just try to execute it again, or execute any query in the same window, like

print getdate(); — or

select getdate();

You will see a popup window with the error message like the following

Input string was not in a correct format. (mscorlib)

This issue does not exist in SSMS V17.0 and certainly not in earlier version of SSMS application as well.