LINQ Concat casting issue?

Question

In its simple form, I have (in a Console app):

// They don't return null, but just for simplicity's sake
PropertyInfo[] GetProperties() { return null; }
FieldInfo[] GetFields() { return null; }
MethodInfo[] GetMethods() { return null; }
MethodInfo[] GetExtensions() { return null; }

And then somewhere else:

var fields = GetFields()
    .Concat<MemberInfo>(GetProperties())
    .Concat<MemberInfo>(GetMethods().Where(m => !m.IsSpecialName))
    .Concat<MemberInfo>(GetExtensions());

This works (compiles fine). However in my actual case which is similar, it doesn't compile (I'm using Mono here - not in a Console app):

fields = target.GetFields(returnType, flags)
    .Concat<MemberInfo>(target.GetProperties(returnType, flags))
    .Concat<MemberInfo>(target.GetMethods(returnType, Type.EmptyTypes, flags, false)
                                          .Where(m => !m.IsSpecialName))
    .Concat<MemberInfo>(extensionMethods)
    .Select(m => m.Name)
    .ToArray();

Errors:

Error 'System.Collections.Generic.IEnumerable<System.Reflection.MemberInfo>' does not contain a definition for 'Concat' and the best extension method overload 'System.Linq.Enumerable.Concat<TSource>(System.Collections.Generic.IEnumerable<TSource>, System.Collections.Generic.IEnumerable<TSource>)' has some invalid arguments
Error Argument 2: cannot convert from 'System.Collections.Generic.IEnumerable<System.Reflection.MethodInfo>' to 'System.Collections.Generic.IEnumerable<System.Reflection.MemberInfo>'

However, if I comment the extension methods, and do a ToArray on the Where clause, it doesn't complain (no compile errors) which I think doesn't make sense:

fields = target.GetFields(returnType, flags)
    .Concat<MemberInfo>(target.GetProperties(returnType, flags))
    .Concat<MemberInfo>(target.GetMethods(returnType, Type.EmptyTypes, flags, false)
                                          .Where(m => !m.IsSpecialName).ToArray())
    //.Concat<MemberInfo>(extensionMethods)
    .Select(m => m.Name)
    .ToArray();

If I uncomment it starts complaining (gives the same error) again.

Any ideas what's going on?

EDIT: This really seems to be a Mono thing. I copy-pasted the same sample code from the console app (the code posted first) to my actual environment, I got the same errors.

EDIT: Some details about the runtime:

MonoDevelop-Unity
Version 4.0.1
Installation UUID: dc9fa750-d2af-42c8-9831-b510b08f6d28
Runtime:
    Microsoft .NET 4.0.30319.18408
    GTK 2.24.13
    GTK# (2.12.0.0)

Answer 1

This seems to compile, no idea why though :/

var fields = GetFields()
    .Concat(GetProperties().Cast<MemberInfo>())
    .Concat(GetMethods().Cast<MemberInfo>())
    .Concat(GetExtensions());

If anybody could explain I'd be happy.