diff --git a/lib/api.rb b/lib/api.rb
index 6303e48..f5ba937 100644
--- a/lib/api.rb
+++ b/lib/api.rb
@@ -26,7 +26,7 @@ helpers do
   end
 
   def halt_with_meaningful_response(status, message)
-    halt status, "#{message}. Please read http://fixer.io."
+    halt status, message
   end
 end
 
diff --git a/lib/snapshot.rb b/lib/snapshot.rb
index 42a4e3d..c1fb0a6 100644
--- a/lib/snapshot.rb
+++ b/lib/snapshot.rb
@@ -28,6 +28,7 @@ class Snapshot
   def rebase(rates)
     if base.upcase! != 'EUR'
       denominator = rates.update('EUR' => 1.0).delete(base)
+      raise ArgumentError, 'Invalid base' unless denominator
 
       rates.each do |iso_code, rate|
         rates[iso_code] = round(rate / denominator)
diff --git a/spec/api_spec.rb b/spec/api_spec.rb
index a6de438..a111a2f 100644
--- a/spec/api_spec.rb
+++ b/spec/api_spec.rb
@@ -44,4 +44,9 @@ describe 'the API' do
     get '/'
     last_response.status.must_equal 404
   end
+
+  it 'handles malformed queries' do
+    get 'latest?base=USD?callback=?'
+    last_response.must_be :unprocessable?
+  end
 end