diff --git a/lib/resource_generator.rb b/lib/resource_generator.rb
index 86faade..276e4eb 100644
--- a/lib/resource_generator.rb
+++ b/lib/resource_generator.rb
@@ -28,6 +28,7 @@ class RequiredElementGenerationError < StandardError; end
         }.freeze
       }.freeze
       SIMPLE_QUANTITY_FIELDS = {
+        'Observation' => [:valueQuantity],
         'Range' => [:low, :high],
         'SampledData' => [:origin],
         'Observation::ReferenceRange' => [:low, :high]
@@ -642,15 +643,7 @@ def self.relative_fhir_class_name(model)
       def self.apply_r5_invariants!(resource)
         case resource
         when FHIR::R5::UsageContext
-          ensure_serializable_choice!(
-            resource,
-            'value',
-            'valueCodeableConcept',
-            textonly_codeableconcept(
-              'Generated usage context',
-              namespace: FHIR::R5
-            )
-          )
+          apply_usage_context_invariant!(resource, namespace: FHIR::R5)
         when FHIR::R5::BiologicallyDerivedProduct::Property
           ensure_serializable_choice!(
             resource,
@@ -737,6 +730,18 @@ def self.apply_r5_invariants!(resource)
         resource
       end
 
+      def self.apply_usage_context_invariant!(resource, namespace:)
+        ensure_serializable_choice!(
+          resource,
+          'value',
+          'valueCodeableConcept',
+          textonly_codeableconcept(
+            'Generated usage context',
+            namespace: namespace
+          )
+        )
+      end
+
       def self.ensure_serializable_choice!(resource, prefix, selected_field, value)
         fields = multiple_type_fields(resource.class).fetch(prefix).values
         populated_fields = fields.select do |field|
@@ -794,6 +799,10 @@ def self.ensure_r5_test_script_action!(test)
       def self.apply_invariants!(resource)
         fix_codeable_reference(resource)
         clear_prohibited_observation_quantity_comparators!(resource)
+        apply_usage_context_invariant!(resource, namespace: FHIR) if
+          resource.is_a?(FHIR::UsageContext)
+        apply_usage_context_invariant!(resource, namespace: FHIR::R4B) if
+          resource.is_a?(FHIR::R4B::UsageContext)
         apply_r5_invariants!(resource) if
           Crucible::FHIRVersion.for_class(resource) == :r5
 
diff --git a/test/unit/observation_generation_test.rb b/test/unit/observation_generation_test.rb
index 76d1906..98348ed 100644
--- a/test/unit/observation_generation_test.rb
+++ b/test/unit/observation_generation_test.rb
@@ -35,15 +35,22 @@ def test_observation_simple_quantities_have_no_comparator_across_namespaces
     end
   end
 
-  def test_r4_and_r4b_observation_quantities_that_allow_comparators_are_unchanged
+  def test_r4_and_r4b_observation_quantity_comparators_are_selectively_cleared
     VERSIONS.each do |version, namespace|
       observation = observation_with_allowed_comparators(namespace)
-      expected = allowed_quantities(observation).map { |path, quantity| [path, quantity.comparator] }
 
       Crucible::Tests::ResourceGenerator.apply_invariants!(observation)
 
       actual = allowed_quantities(observation).map { |path, quantity| [path, quantity.comparator] }
-      assert_equal expected, actual, "#{version} cleared an allowed Quantity comparator"
+      expected = [
+        ["valueQuantity", nil],
+        ["valueRatio.numerator", ">"],
+        ["valueRatio.denominator", "<="],
+        ["component[0].valueQuantity", ">="],
+        ["component[0].valueRatio.numerator", "<="],
+        ["component[0].valueRatio.denominator", ">"]
+      ]
+      assert_equal expected, actual, "#{version} applied the wrong Quantity comparator invariant"
     end
   end
 
diff --git a/test/unit/resource_generator_test.rb b/test/unit/resource_generator_test.rb
index dbd68f8..2bcc71d 100644
--- a/test/unit/resource_generator_test.rb
+++ b/test/unit/resource_generator_test.rb
@@ -47,6 +47,47 @@ def test_empty_r4b_codeable_reference_gets_a_concept
     assert_nil reference.reference
   end
 
+  def test_empty_r4_usage_context_gets_a_value
+    context = FHIR::UsageContext.new
+
+    Crucible::Tests::ResourceGenerator.apply_invariants!(context)
+
+    assert_instance_of FHIR::CodeableConcept, context.valueCodeableConcept
+    assert_not_empty context.valueCodeableConcept.text
+    assert_nil context.valueQuantity
+    assert_nil context.valueRange
+    assert_nil context.valueReference
+  end
+
+  def test_empty_r4b_usage_context_gets_a_value
+    context = FHIR::R4B::UsageContext.new
+
+    Crucible::Tests::ResourceGenerator.apply_invariants!(context)
+
+    assert_instance_of FHIR::R4B::CodeableConcept, context.valueCodeableConcept
+    assert_not_empty context.valueCodeableConcept.text
+    assert_nil context.valueQuantity
+    assert_nil context.valueRange
+    assert_nil context.valueReference
+  end
+
+  def test_observation_value_quantity_comparator_is_cleared_for_supported_versions
+    [
+      [FHIR::Observation, FHIR::Quantity],
+      [FHIR::R4B::Observation, FHIR::R4B::Quantity],
+      [FHIR::R5::Observation, FHIR::R5::Quantity]
+    ].each do |observation_class, quantity_class|
+      quantity = quantity_class.new(value: 10, comparator: '>')
+      observation = observation_class.new(valueQuantity: quantity)
+
+      Crucible::Tests::ResourceGenerator.apply_invariants!(observation)
+
+      assert_instance_of quantity_class, observation.valueQuantity
+      assert_equal 10, observation.valueQuantity.value
+      assert_nil observation.valueQuantity.comparator
+    end
+  end
+
   def test_populated_r4b_codeable_reference_is_preserved
     reference = FHIR::R4B::CodeableReference.new
     reference.reference = FHIR::R4B::Reference.new(display: 'Existing reference')