When a constant method-level [RowCountHint(...)] is specified, it must include a positive integer to use as the estimated row count.

Bad:

[RowCountHint]
public void SomeMethod()
{ ... }

Also bad:

[RowCountHint(-15)]
public void SomeMethod()
{ ... }

Good:

[RowCountHint(100)]
public void SomeMethod()
{ ... }