diff --git a/gseapy/biomart.py b/gseapy/biomart.py
index 8f4405b..928e6ca 100644
--- a/gseapy/biomart.py
+++ b/gseapy/biomart.py
@@ -41,7 +41,7 @@ def __init__(self, host="www.ensembl.org", verbose=False):
 
         self.attributes_xml = []
         self.filters_xml = []
-        self.dataset_xml = None
+        self.dataset_xml = ""
 
         params = {
             "version": "1.0",
@@ -86,7 +86,7 @@ def add_dataset(self, dataset):
     def reset(self):
         self.attributes_xml = []
         self.filters_xml = []
-        self.dataset_xml = None
+        self.dataset_xml = ""
 
     def get_xml(self):
         xml = self.header
@@ -178,7 +178,7 @@ def query(
                 v = ",".join(list(v))
                 self.add_filter_to_xml(k, v)
 
-        xml_query = self.get_xml()
+        xml_query = super(Biomart, self).get_xml()
         results = super(Biomart, self).query(xml_query)
         if str(results).startswith("Query ERROR"):
             print(results)