select many error

Dim x = countDictionary.SelectMany(Function(o) o.Key.MasterCategory)

o.Key As Category

Results in:

Overload resolution failed because no accessible 'SelectMany' can be called with these arguments:
    Extension method 'Public Function SelectMany(Of TResult)(selector As System.Func(Of System.Collections.Generic.KeyValuePair(Of Category, System.Collections.Generic.Dictionary(Of Date, Integer)), Integer, System.Collections.Generic.IEnumerable(Of TResult))) As System.Collections.Generic.IEnumerable(Of TResult)' defined in 'System.Linq.Enumerable': Nested function does not have a signature that is compatible with delegate 'System.Func(Of System.Collections.Generic.KeyValuePair(Of Category, System.Collections.Generic.Dictionary(Of Date, Integer)), Integer, System.Collections.Generic.IEnumerable(Of TResult))'.

    Extension method 'Public Function SelectMany(Of TResult)(selector As System.Func(Of System.Collections.Generic.KeyValuePair(Of Category, System.Collections.Generic.Dictionary(Of Date, Integer)), Integer, System.Collections.Generic.IEnumerable(Of TResult))) As System.Collections.Generic.IEnumerable(Of TResult)' defined in 'System.Linq.Enumerable': Data type(s) of the type parameter(s) cannot be inferred from these arguments. Specifying the data type(s) explicitly might correct this error.

    Extension method 'Public Function SelectMany(Of TResult)(selector As System.Func(Of System.Collections.Generic.KeyValuePair(Of Category, System.Collections.Generic.Dictionary(Of Date, Integer)), System.Collections.Generic.IEnumerable(Of TResult))) As System.Collections.Generic.IEnumerable(Of TResult)' defined in 'System.Linq.Enumerable': Data type(s) of the type parameter(s) cannot be inferred from these arguments. Specifying the data type(s) explicitly might correct this error.