diff --git a/python_ifirma/core.py b/python_ifirma/core.py
index 223a87e..a318770 100644
--- a/python_ifirma/core.py
+++ b/python_ifirma/core.py
@@ -69,10 +69,12 @@ class Position:
 
 
 class NewInvoiceParams:
-    def __init__(self, client, positions):
+    def __init__(self, client, positions, number=None,
+                 issue_date=datetime.date.today()):
         self.client = client
         self.positions = positions
-        self.issue_date = datetime.date.today()
+        self.number = number
+        self.issue_date = issue_date
 
     def __get_issue_date(self):
         return strftime("%Y-%m-%d", self.issue_date.timetuple())
@@ -87,6 +89,7 @@ class NewInvoiceParams:
         )
 
     def get_request_data(self):
+        series = self.number.split('/')[0][-1]
         return {
             "Zaplacono": self.__get_total_price(),
             "ZaplaconoNaDokumencie": self.__get_total_price(),
@@ -98,6 +101,7 @@ class NewInvoiceParams:
             "RodzajPodpisuOdbiorcy": "BPO",
             "WidocznyNumerGios": False,
             "Numer": None,
+            "NazwaSeriiNumeracji": series,
             "Pozycje": [position.get_dict() for position in self.positions],
             "Kontrahent": self.client.get_dict(),
         }
@@ -126,6 +130,8 @@ class iFirmaAPI():
         response_code = real_response_content.get("Kod", -1)
 
         if response_code != 0:
+            print(response_code)
+            print(real_response_content)
             raise PythonIfirmaExceptionFactory.throw_exception_by_code(response_code)
 
         return response_dict
@@ -137,6 +143,8 @@ class iFirmaAPI():
         )
 
     def __create_invoice_and_return_id(self, invoice, url):
+        from pprint import pprint
+        pprint(invoice.get_request_data())
         request_content = json.dumps(invoice.get_request_data(), separators=(',', ':'))
         request_hash_text = "{}{}{}{}".format(
             url,
@@ -202,4 +210,4 @@ class iFirmaAPI():
         file_obj = six.BytesIO()
         file_obj.write(content)
         file_obj.seek(0)
-        return file_obj
\ No newline at end of file
+        return file_obj