EF Core Missing Method HasIndex

I've just started to migrate to a SQL database and running into a problem with a MissingMethodException being thrown. Here is the configuration class that is throwing the error:

using System;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata.Builders;
using Project.Core.Entities.Users;

namespace Project.Persistance.Configuration.Users
{
    public class UserClaimTypeConfiguration : IEntityTypeConfiguration<UserClaimType>
    {
        public void Configure(EntityTypeBuilder<UserClaimType> builder)
        {
            builder.HasKey(entity => entity.Id);

            builder.Property(entity => entity.Name)
                        .IsRequired()
                        .HasMaxLength(30);

            builder.HasIndex(entity => entity.Name);

            builder.Property(entity => entity.Description)
                        .IsRequired(false)
                        .HasMaxLength(100);

            builder.Ignore(entity => entity.ValueType);
        }
    }
}

And this is error I am getting in the console:

PM> add-migration user
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.MissingMethodException: Method not found: 'Microsoft.EntityFrameworkCore.Metadata.Builders.IndexBuilder Microsoft.EntityFrameworkCore.Metadata.Builders.EntityTypeBuilder`1.HasIndex(System.Linq.Expressions.Expression`1<System.Func`2<!0,System.Object>>)'.
   at Project.Persistance.Configuration.Users.UserClaimTypeConfiguration.Configure(EntityTypeBuilder`1 builder)
   at Microsoft.EntityFrameworkCore.ModelBuilder.ApplyConfiguration[TEntity](IEntityTypeConfiguration`1 configuration)

I've tried 'Goggle' and the Microsoft docs but can't seem to find any reference to this issue - so it must be my setup. Just can't figure out what is causing it!