diff --git a/src/Perspex.Base/BindingDescriptor.cs b/src/Perspex.Base/BindingDescriptor.cs
index 4ee4966a98..c0c8f3406f 100644
--- a/src/Perspex.Base/BindingDescriptor.cs
+++ b/src/Perspex.Base/BindingDescriptor.cs
@@ -78,6 +78,18 @@ namespace Perspex
set;
}
+ ///
+ /// Gets or sets the source observable.
+ ///
+ ///
+ /// If null, then . will be used.
+ ///
+ public IObservable