diff --git a/lib/fortnox/api/types.rb b/lib/fortnox/api/types.rb
index ce9c2ef6..6798e9d6 100644
--- a/lib/fortnox/api/types.rb
+++ b/lib/fortnox/api/types.rb
@@ -38,6 +38,15 @@ module Types
       AccountNumber = Strict::Int
                       .constrained(gteq: 0, lteq: 9999)
                       .optional
+                      .constructor do |input|
+                        unless input.nil? || input.to_s.empty?
+                          Integer(input)
+                        else
+                          input
+                        end
+                      rescue ArgumentError
+                        input
+                      end
 
       ArticleType = Strict::String
                     .constrained(included_in: ArticleTypes.values)
diff --git a/spec/fortnox/api/types/account_number_spec.rb b/spec/fortnox/api/types/account_number_spec.rb
index f5d6af46..e7d5e84c 100644
--- a/spec/fortnox/api/types/account_number_spec.rb
+++ b/spec/fortnox/api/types/account_number_spec.rb
@@ -28,4 +28,16 @@
   context 'when AccountNumber created with a negative number' do
     include_examples 'raises ConstraintError', -1
   end
+
+  context 'when AccountNumber created with an invalid string' do
+    include_examples 'raises ConstraintError', 'foo'
+  end
+
+  context 'when AccountNumber created with valid string' do
+    subject { klass['1234'] }
+
+    it 'casts it to a number' do
+      is_expected.to eq 1234
+    end
+  end
 end