diff --git a/lib/signature.rb b/lib/signature.rb
index 00bc576..7b1b477 100644
--- a/lib/signature.rb
+++ b/lib/signature.rb
@@ -82,6 +82,7 @@ def authenticate_by_token(token, timestamp_grace = 600)
     end
 
     def authenticate(timestamp_grace = 600, &block)
+      raise ArgumentError, "Block required" unless block_given?
       key = @auth_hash['auth_key']
       raise AuthenticationError, "Authentication key required" unless key
       token = yield key
diff --git a/spec/signature_spec.rb b/spec/signature_spec.rb
index 3167669..918974e 100644
--- a/spec/signature_spec.rb
+++ b/spec/signature_spec.rb
@@ -189,6 +189,13 @@
           request.authenticate { |key| nil }
         }.should raise_error('Invalid authentication key')
       end
+
+      it "should raise unless block given" do
+        request = Signature::Request.new('POST', '/some/path', @params)
+        lambda {
+          request.authenticate
+        }.should raise_error(ArgumentError, "Block required")
+      end
     end
   end
 end