diff --git a/lib/fhir_client/ext/reference.rb b/lib/fhir_client/ext/reference.rb
index a9e90163..9e7a95d2 100644
--- a/lib/fhir_client/ext/reference.rb
+++ b/lib/fhir_client/ext/reference.rb
@@ -82,7 +82,7 @@ class Reference
     include FHIR::ReferenceExtras
 
     def resource_class
-      "FHIR::#{resource_type}".constantize unless contained?
+      FHIR.const_get(resource_type).constantize unless contained?
     end
   end
 end
@@ -93,7 +93,7 @@ class Reference
       include FHIR::ReferenceExtras
 
       def resource_class
-        "FHIR::DSTU2::#{resource_type}".constantize unless contained?
+        FHIR::DSTU2.const_get(resource_type).constantize unless contained?
       end
     end
   end
@@ -105,7 +105,7 @@ class Reference
       include FHIR::ReferenceExtras
 
       def resource_class
-        "FHIR::STU3::#{resource_type}".constantize unless contained?
+        FHIR::STU3.const_get(resource_type).constantize unless contained?
       end
     end
   end
@@ -117,7 +117,7 @@ class Reference < FHIR::R4B::Model
       include FHIR::ReferenceExtras
 
       def resource_class
-        "FHIR::R4B::#{resource_type}".constantize unless contained?
+        FHIR::R4B.const_get(resource_type).constantize unless contained?
       end
     end
   end
@@ -129,7 +129,7 @@ class Reference < FHIR::R5::Model
       include FHIR::ReferenceExtras
 
       def resource_class
-        "FHIR::R5::#{resource_type}".constantize unless contained?
+        FHIR::R5.const_get(resource_type).constantize unless contained?
       end
     end
   end
diff --git a/lib/fhir_client/version_management.rb b/lib/fhir_client/version_management.rb
index 7996fc35..7a8c22ea 100644
--- a/lib/fhir_client/version_management.rb
+++ b/lib/fhir_client/version_management.rb
@@ -8,9 +8,9 @@ def versioned_resource_class(klass = nil)
             when :dstu2
               FHIR::DSTU2
             when :r4b
-              defined?(FHIR::R4B) ? FHIR::R4B : FHIR
+              FHIR::R4B
             when :r5
-              defined?(FHIR::R5) ? FHIR::R5 : FHIR
+              FHIR::R5
             else
               FHIR
             end