diff --git a/lib/rackstash.rb b/lib/rackstash.rb
index 60dae1e..6001537 100644
--- a/lib/rackstash.rb
+++ b/lib/rackstash.rb
@@ -75,6 +75,7 @@ module Rackstash
 
   def self.severity_label(severity)
     if severity.is_a?(Integer)
+      return SEVERITY_LABELS.last if severity < 0
       SEVERITY_LABELS[severity] || SEVERITY_LABELS.last
     else
       severity = SEVERITY_NAMES.fetch(severity.to_s.downcase, UNKNOWN)
diff --git a/spec/rackstash_spec.rb b/spec/rackstash_spec.rb
index 7da0266..3a716a6 100644
--- a/spec/rackstash_spec.rb
+++ b/spec/rackstash_spec.rb
@@ -57,4 +57,30 @@ describe Rackstash do
     expect(Rackstash::UNDEFINED).not_to eql true
     expect(Rackstash::UNDEFINED).not_to eql 42
   end
+
+  describe '.severity_label' do
+    it 'returns the label for an integer severity' do
+      expect(described_class.severity_label(0)).to eql 'DEBUG'
+      expect(described_class.severity_label(4)).to eql 'FATAL'
+      expect(described_class.severity_label(5)).to eql 'ANY'
+    end
+
+    it 'returns ANY for out-of-range severities' do
+      expect(described_class.severity_label(-3)).to eql 'ANY'
+      expect(described_class.severity_label(42)).to eql 'ANY'
+    end
+
+    it 'returns the label for a named severity' do
+      expect(described_class.severity_label('DeBuG')).to eql 'DEBUG'
+      expect(described_class.severity_label('warn')).to eql 'WARN'
+      expect(described_class.severity_label(:error)).to eql 'ERROR'
+      expect(described_class.severity_label('UnknoWn')).to eql 'ANY'
+    end
+
+    it 'returns ANY for unknown severity names' do
+      expect(described_class.severity_label('foo')).to eql 'ANY'
+      expect(described_class.severity_label(:test)).to eql 'ANY'
+      expect(described_class.severity_label(nil)).to eql 'ANY'
+    end
+  end
 end