diff --git a/disciplines/units/specialunits.ttl b/disciplines/units/specialunits.ttl
index d26a476c..28328996 100644
--- a/disciplines/units/specialunits.ttl
+++ b/disciplines/units/specialunits.ttl
@@ -6,6 +6,7 @@
 @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
 @prefix skos: <http://www.w3.org/2004/02/skos/core#> .
 @prefix dcterms: <http://purl.org/dc/terms/> .
+@prefix emmo: <https://w3id.org/emmo#> .
 @base <https://w3id.org/emmo#> .
 
 <https://w3id.org/emmo/disciplines/units/specialunits> rdf:type owl:Ontology ;