diff --git a/structuredefinitions/UKCore-DiagnosticReport.xml b/structuredefinitions/UKCore-DiagnosticReport.xml
index 34e860e7..4a7c7d69 100644
--- a/structuredefinitions/UKCore-DiagnosticReport.xml
+++ b/structuredefinitions/UKCore-DiagnosticReport.xml
@@ -34,7 +34,7 @@
         <severity value="error" />
         <human value="Only one of DiagnosticReport.media.link or DiagnosticReport.media.extension:mediaLinkR5 can be populated at a time." />
         <expression
-          value="(media.link.exists() and not extension('http://hl7.org/fhir/5.0/StructureDefinition/extension-DiagnosticReport.media.link').exists()) or (not media.link.exists() and extension('http://hl7.org/fhir/5.0/StructureDefinition/extension-DiagnosticReport.media.link').exists())" />
+          value="(media.link.exists() and not DiagnosticReport.extension:mediaLinkR5('http://hl7.org/fhir/5.0/StructureDefinition/extension-DiagnosticReport.media.link').exists()) or (not media.link.exists() and DiagnosticReport.extension:mediaLinkR5('http://hl7.org/fhir/5.0/StructureDefinition/extension-DiagnosticReport.media.link').exists())" />
       </constraint>
     </element>
     <element id="DiagnosticReport.extension:compositionReferenceR5">