diff --git a/ogn/commands/bulkimport.py b/ogn/commands/bulkimport.py
index 80e77d3..9d5c2ae 100644
--- a/ogn/commands/bulkimport.py
+++ b/ogn/commands/bulkimport.py
@@ -36,7 +36,7 @@ def convert(sourcefile, path=''):
     match = re.search('^.+\.txt\_(\d{4}\-\d{2}\-\d{2})(\.gz)?$', sourcefile)
     if match:
         reference_date_string = match.group(1)
-        reference_date = datetime.strptime(reference_date_string, "%Y-%m-%d")
+        reference_date = datetime.datetime.strptime(reference_date_string, "%Y-%m-%d")
     else:
         print("filename '{}' does not match pattern. Skipping".format(sourcefile))
         return
diff --git a/ogn/model/aircraft_beacon.py b/ogn/model/aircraft_beacon.py
index a389f38..f0eea61 100644
--- a/ogn/model/aircraft_beacon.py
+++ b/ogn/model/aircraft_beacon.py
@@ -94,7 +94,7 @@ class AircraftBeacon(Beacon):
     def get_csv_values(self):
         return [
             self.location_wkt,
-            int(self.altitude),
+            int(self.altitude) if self.altitude else None,
             self.name,
             self.receiver_name,
             self.timestamp,