c# - Editable Combobox with DisplayMemberPath / SelectedValuePath -

i have combobox in wpf:

<combobox displaymemberpath="value"           iseditable="true"           itemssource="{binding descriptioncodes}"           selectedvalue="{binding descriptioncode, mode=twoway}"           selectedvaluepath="key" /> 

descriptioncodes dictionary<string, string>, codes matched friendly names. user has ability enter own code (as odd sounds, that's 'norm' of our application suite, , intention).

my binding however, creates scenario if user types in value, descriptioncode becomes null, value user intputted doesn't exist in dictionary<string, string>.

is there way working way want?