C# XML can't resolve entity, yet it exists

I'm parsing an XML file that contains entities. I receive the following exception:

Reference to undeclared entity 'deg'. Line 283, position 53.

This is my DOCTYPE:

<!DOCTYPE dmodule [
<!ENTITY % ISOEntities PUBLIC "ISO 8879-1986//ENTITIES ISO Character Entities 20030531//EN//XML" "ent/ISOEntities">
%ISOEntities;
]>

Inside the ISOEntities file I have this line:

<!ENTITY deg    "&#x00B0;"> <!-- DEGREE SIGN -->

The entity definition exists. I don't understand why this particular entity is causing an error, when everything else is being resolved.