diff --git a/lib/rackstash/class_registry.rb b/lib/rackstash/class_registry.rb
index 953f45e..a3dff1e 100644
--- a/lib/rackstash/class_registry.rb
+++ b/lib/rackstash/class_registry.rb
@@ -25,17 +25,40 @@ module Rackstash
     #
     # @param spec [Class,String,Symbol] Either a class (in which case it is
     #   returned directly) or the name of a registered class.
-    # @raise [KeyError] when giving a `String` or `Symbol` but no registered
-    #   class was found for it
     # @raise [TypeError] when giving an invalid object
+    # @return [Class, nil] the registered class (when giving a `String` or
+    #   `Symbol`) or the given class (when giving a `Class`) or `nil` if no
+    #   registered class could be found
+    # @see #fetch
+    def [](spec)
+      fetch(spec, nil)
+    end
+
+    # Retrieve the registered class for a given name. If the argument is already
+    # a class, we return it unchanged.
+    #
+    # @param spec [Class,String,Symbol] either a class (in which case it is
+    #   returned directly) or the name of a registered class
+    # @param default [Object] the default value that is returned if no
+    #   registered class could be found for the given `spec` and no block was
+    #   given.
+    # @yield if no registered class could be found for the given `spec`, we will
+    #   run the optional block and return its result
+    # @yieldparam spec [Symbol] the requested class specification as a `Symbol`
+    # @raise [KeyError] when giving a `String` or `Symbol` but no registered
+    #   class was found for it and no default was specified
+    # @raise [TypeError] when giving an invalid `spec` object
     # @return [Class] the registered class (when giving a `String` or `Symbol`)
     #   or the given class (when giving a `Class`)
-    def [](spec)
+    def fetch(spec, default = UNDEFINED)
       case spec
       when Class
         spec
       when String, Symbol, ->(s) { s.respond_to?(:to_sym) }
-        @registry.fetch(spec.to_sym) do
+        @registry.fetch(spec.to_sym) do |key|
+          next yield(key) if block_given?
+          next default unless UNDEFINED.equal? default
+
           raise KeyError, "No #{@object_type} was registered for #{spec.inspect}"
         end
       else
diff --git a/lib/rackstash/encoder.rb b/lib/rackstash/encoder.rb
index 405752e..84f6956 100644
--- a/lib/rackstash/encoder.rb
+++ b/lib/rackstash/encoder.rb
@@ -81,7 +81,7 @@ module Rackstash
         if encoder_spec.respond_to?(:encode) && !encoder_spec.is_a?(String)
           encoder_spec
         else
-          registry[encoder_spec].new(*args, &block)
+          registry.fetch(encoder_spec).new(*args, &block)
         end
       end
     end
diff --git a/lib/rackstash/filter.rb b/lib/rackstash/filter.rb
index e160980..2cd7ecd 100644
--- a/lib/rackstash/filter.rb
+++ b/lib/rackstash/filter.rb
@@ -94,7 +94,7 @@ module Rackstash
         else
           args << kwargs unless kwargs.empty?
 
-          filter = registry[filter_spec].new(*args, &block)
+          filter = registry.fetch(filter_spec).new(*args, &block)
           conditional_filter(filter, only_if: only_if, not_if: not_if)
         end
       end
diff --git a/spec/rackstash/class_registry_spec.rb b/spec/rackstash/class_registry_spec.rb
index cb21cbc..b3c68d2 100644
--- a/spec/rackstash/class_registry_spec.rb
+++ b/spec/rackstash/class_registry_spec.rb
@@ -37,19 +37,53 @@ describe Rackstash::ClassRegistry do
       expect(registry[String]).to equal String
     end
 
+    it 'returns nil on unknown names' do
+      expect(registry[:unknown]).to be nil
+      expect(registry['invalid']).to be nil
+    end
+  end
+
+  describe '#fetch' do
+    before do
+      registry[:class] = klass
+    end
+
+    it 'returns the class for a String' do
+      expect(registry.fetch('class')).to equal klass
+    end
+
+    it 'returns the class for a Symbol' do
+      expect(registry.fetch(:class)).to equal klass
+    end
+
+    it 'returns an actual class' do
+      expect(registry.fetch(klass)).to equal klass
+      expect(registry.fetch(String)).to equal String
+    end
+
     it 'raises a KeyError on unknown names' do
-      expect { registry[:unknown] }
+      expect { registry.fetch(:unknown) }
         .to raise_error(KeyError, 'No value was registered for :unknown')
-      expect { registry['invalid'] }
+      expect { registry.fetch('invalid') }
         .to raise_error(KeyError, 'No value was registered for "invalid"')
     end
 
+    it 'returns the default value with unknown names' do
+      expect(registry.fetch(:unknown, 'Hello World')).to eql 'Hello World'
+      expect(registry.fetch('invalid', 'Hello World')).to eql 'Hello World'
+    end
+
+    it 'calls the block and returns its value with unknown names' do
+      expect { |b| registry.fetch('invalid', &b) }.to yield_with_args(:invalid)
+      expect(registry.fetch('invalid') { |e| e.upcase }).to eql :INVALID
+    end
+
     it 'raises a TypeError on invalid names' do
-      expect { registry[0] }
+      expect { registry.fetch(0) }
         .to raise_error(TypeError, '0 can not be used to describe values')
-      expect { registry[nil] }
+      expect { registry.fetch(nil) }
         .to raise_error(TypeError, 'nil can not be used to describe values')
-      expect { registry[true] }
+      expect { registry.fetch(true) }
         .to raise_error(TypeError, 'true can not be used to describe values')
     end
   end
@@ -88,7 +122,7 @@ describe Rackstash::ClassRegistry do
       expect(registry[:class]).to equal klass
 
       expect(registry.clear).to equal registry
-      expect { registry[:class] }.to raise_error(KeyError)
+      expect(registry[:class]).to be nil
     end
   end
 
@@ -147,7 +181,7 @@ describe Rackstash::ClassRegistry do
       hash = registry.to_h
       hash[:alias] = klass
 
-      expect { registry[:alias] }.to raise_error(KeyError)
+      expect(registry[:alias]).to be nil
     end
   end
 end