diff --git a/lib/prometheus/client/label_set_validator.rb b/lib/prometheus/client/label_set_validator.rb
index f3625233..e67dde54 100644
--- a/lib/prometheus/client/label_set_validator.rb
+++ b/lib/prometheus/client/label_set_validator.rb
@@ -5,8 +5,7 @@ module Client
     # LabelSetValidator ensures that all used label sets comply with the
     # Prometheus specification.
     class LabelSetValidator
-      # TODO: we might allow setting :instance in the future
-      BASE_RESERVED_LABELS = [:job, :instance, :pid].freeze
+      BASE_RESERVED_LABELS = [:pid].freeze
       LABEL_NAME_REGEX = /\A[a-zA-Z_][a-zA-Z0-9_]*\Z/
 
       class LabelSetError < StandardError; end
diff --git a/spec/prometheus/client/label_set_validator_spec.rb b/spec/prometheus/client/label_set_validator_spec.rb
index f4d38987..ffd1387c 100644
--- a/spec/prometheus/client/label_set_validator_spec.rb
+++ b/spec/prometheus/client/label_set_validator_spec.rb
@@ -4,7 +4,10 @@
 
 describe Prometheus::Client::LabelSetValidator do
   let(:expected_labels) { [] }
-  let(:validator) { Prometheus::Client::LabelSetValidator.new(expected_labels: expected_labels) }
+  let(:additional_reserved_labels) { [] }
+  let(:validator) do
+    Prometheus::Client::LabelSetValidator.new(expected_labels: expected_labels, reserved_labels: additional_reserved_labels)
+  end
   let(:invalid) { Prometheus::Client::LabelSetValidator::InvalidLabelSetError }
 
   describe '.new' do
@@ -42,11 +45,27 @@
       end.to raise_exception(described_class::InvalidLabelError)
     end
 
-    it 'raises ReservedLabelError if a label key is reserved' do
-      [:job, :instance, :pid].each do |label|
-        expect do
-          validator.validate_symbols!(label => 'value')
-        end.to raise_exception(described_class::ReservedLabelError)
+    context "with only the base set of reserved labels" do
+      it "doesn't raise ReservedLabelError for the additional reserved label" do
+        expect { validator.validate_symbols!(additional: 'value') }.
+          to_not raise_exception
+      end
+
+      it 'raises ReservedLabelError if a label key is reserved' do
+        expect { validator.validate_symbols!(pid: 'value') }.
+          to raise_exception(described_class::ReservedLabelError)
+      end
+    end
+
+    context "with an additional reserved label" do
+      let(:additional_reserved_labels) { [:additional] }
+
+      it 'raises ReservedLabelError if a label key is reserved' do
+        [:additional, :pid].each do |label|
+          expect do
+            validator.validate_symbols!(label => 'value')
+          end.to raise_exception(described_class::ReservedLabelError)
+        end
       end
     end
   end