Why am I getting a contentRootPath null exception when attempting to add a database migration in entityframework core?

I have a .NET Core class library that is the data access layer for multiple web applications. I am using Entity Framework Core 1.1, but I'm having trouble with migrations through PMC. My initial migration worked successfully, but after updating the database and re-launching VS 2017 I get a null exception on the contentRootPath argument when attempting to migrate. I may have made some changes to the project after my successful migration, but I don't remember what they were. This is the error and stack trace:

Both Entity Framework Core and Entity Framework 6 are installed. The Entity Framework Core tools are running. Use 'EntityFramework\Add-Migration' for Entity Framework 6.
System.ArgumentNullException: Value cannot be null.
Parameter name: contentRootPath
   at Microsoft.EntityFrameworkCore.Utilities.Check.NotEmpty(String value, String parameterName)
   at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations..ctor(IOperationReporter reporter, Assembly assembly, Assembly startupAssembly, String environment, String projectDir, String contentRootPath, String rootNamespace)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.<>c__DisplayClass4_0.<.ctor>b__4()
   at Microsoft.EntityFrameworkCore.Internal.LazyRef`1.get_Value()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType)
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
   at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)

There is no config file in my migrations directory, and I can't find anything related in the DbContext class. Has anyone else ran into this issue, or know what is causing it?

SqlDbContext class:

using Microsoft.EntityFrameworkCore;
using DataLayer.Models.User;

namespace DataLayer.Context 
{
  public class SqlDbContext : DbContext
  {
      public DbSet<User> Users { get; set; }

      protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
      {
          optionsBuilder.UseSqlServer(MyDbConfig.SqlConnectionString);   
      }

      protected override void OnModelCreating(ModelBuilder modelBuilder)
      {
          modelBuilder.Entity<User>()
              .Ignore(m => m.FullName);

          base.OnModelCreating(modelBuilder);
      }
   }
}