diff --git a/lib/rackstash/encoder.rb b/lib/rackstash/encoder.rb
index dc472be..405752e 100644
--- a/lib/rackstash/encoder.rb
+++ b/lib/rackstash/encoder.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 #
-# Copyright 2017 Holger Just
+# Copyright 2017 - 2018 Holger Just
 #
 # This software may be modified and distributed under the terms
 # of the MIT license. See the LICENSE.txt file for details.
@@ -31,12 +31,13 @@ module Rackstash
       # encoder object for it.
       #
       # @param encoder_class [Class] a class from which a new encoder can be
-      #   created. Filter objects must respond to `encode` and accept an event
-      #   hash.
+      #   created. Encoder classes must implement the `encode` instance method
+      #   which accepts an event hash and returns encoded event.
       # @param names [Array<String,Symbol>] one or more names for the
       #   registered `encoder_class`. Using these names, the user can create a
       #   new encoder object from the registered class in {.build}.
-      # @raise [TypeError] if objects of type were passed
+      # @raise [TypeError] if invalid arguments were passed, e.g. an unsuitable
+      #   class or invalid names
       # @return [Class] the passed `filter_class`
       def register(encoder_class, *names)
         unless encoder_class.is_a?(Class) &&
diff --git a/lib/rackstash/filter.rb b/lib/rackstash/filter.rb
index 6bde9be..e160980 100644
--- a/lib/rackstash/filter.rb
+++ b/lib/rackstash/filter.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 #
-# Copyright 2017 Holger Just
+# Copyright 2017 - 2018 Holger Just
 #
 # This software may be modified and distributed under the terms
 # of the MIT license. See the LICENSE.txt file for details.
@@ -29,14 +29,20 @@ module Rackstash
       # object for it.
       #
       # @param filter_class [Class] a class from which a new filter can be
-      #   created. Filter objects must respond to `call` and accept an event
-      #   hash.
+      #   created. Filter classes must implement the `call` instance method
+      #   which accepts an event hash.
       # @param filter_names [Array<String,Symbol>] one or more names for the
       #   registered `filter_class`. Using these names, the user can create a
       #   new filter object from the registered class in {.build}.
-      # @raise [TypeError] if objects of type were passed
+      # @raise [TypeError] if invalid arguments were passed, e.g. an unsuitable
+      #   class or invalid names
       # @return [Class] the passed `filter_class`
       def register(filter_class, *filter_names)
+        unless filter_class.is_a?(Class) &&
+               filter_class.instance_methods.include?(:call)
+          raise TypeError, 'Can only register filter classes'
+        end
+
         filter_names.flatten.each do |name|
           registry[name] = filter_class
         end
diff --git a/spec/rackstash/encoder_spec.rb b/spec/rackstash/encoder_spec.rb
index 613a0a1..be319c9 100644
--- a/spec/rackstash/encoder_spec.rb
+++ b/spec/rackstash/encoder_spec.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 #
-# Copyright 2017 Holger Just
+# Copyright 2017 - 2018 Holger Just
 #
 # This software may be modified and distributed under the terms
 # of the MIT license. See the LICENSE.txt file for details.
@@ -79,10 +79,36 @@ describe Rackstash::Encoder do
     end
   end
 
-  describe 'registry' do
+  describe '.registry' do
     it 'returns the encoder registry' do
       expect(described_class.registry).to be_instance_of Rackstash::ClassRegistry
       expect(described_class.registry.object_type).to eql 'encoder'
     end
   end
+
+  describe '.register' do
+    let(:encoder_class) {
+      Class.new do
+        def encode; end
+      end
+    }
+
+    it 'registers an encoder class' do
+      expect(described_class.registry).to receive(:[]=).with(:foo, encoder_class).ordered
+      expect(described_class.registry).to receive(:[]=).with(:bar, encoder_class).ordered
+
+      described_class.register(encoder_class, :foo, :bar)
+    end
+
+    it 'rejects invalid classes' do
+      expect(described_class.registry).not_to receive(:[]=)
+
+      expect { described_class.register(:not_a_class, :foo) }.to raise_error TypeError
+      expect { described_class.register(Class.new, :foo) }.to raise_error TypeError
+    end
+
+    it 'rejects invalid names' do
+      expect { described_class.register(encoder_class, 123) }.to raise_error TypeError
+    end
+  end
 end
diff --git a/spec/rackstash/filter_spec.rb b/spec/rackstash/filter_spec.rb
index a11557d..839d763 100644
--- a/spec/rackstash/filter_spec.rb
+++ b/spec/rackstash/filter_spec.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 #
-# Copyright 2017 Holger Just
+# Copyright 2017 - 2018 Holger Just
 #
 # This software may be modified and distributed under the terms
 # of the MIT license. See the LICENSE.txt file for details.
@@ -150,10 +150,36 @@ describe Rackstash::Filter do
     end
   end
 
-  describe 'registry' do
+  describe '.registry' do
     it 'returns the filter registry' do
       expect(described_class.registry).to be_instance_of Rackstash::ClassRegistry
       expect(described_class.registry.object_type).to eql 'filter'
     end
   end
+
+  describe '.register' do
+    let(:filter_class) {
+      Class.new do
+        def call; end
+      end
+    }
+
+    it 'registers a filter class' do
+      expect(described_class.registry).to receive(:[]=).with(:foo, filter_class).ordered
+      expect(described_class.registry).to receive(:[]=).with(:bar, filter_class).ordered
+
+      described_class.register(filter_class, :foo, :bar)
+    end
+
+    it 'rejects invalid classes' do
+      expect(described_class.registry).not_to receive(:[]=)
+
+      expect { described_class.register(:not_a_class, :foo) }.to raise_error TypeError
+      expect { described_class.register(Class.new, :foo) }.to raise_error TypeError
+    end
+
+    it 'rejects invalid names' do
+      expect { described_class.register(filter_class, 123) }.to raise_error TypeError
+    end
+  end
 end