Missing Handler Error On Lambda But Handler Exists In Script

My lambda_function.py file looks like this:

from urllib.request import urlopen
from google.cloud import bigquery
import json

url = "https://data2.unhcr.org/population/get/timeseries?widget_id=286725&sv_id=54&population_group=5460&frequency=day&fromDate=1900-01-01"
bq_client = bigquery.Client()

def lambda_helper(event, context):
    response = urlopen(url)
    data_json = json.loads(response.read())
    bq_client.load_table_from_json(data_json['data']['timeseries'], "xxx.xxxx.tablename")

But every time I zip it up and upload it to my Lambda I get this error:

{
  "errorMessage": "Handler 'lambda_handler' missing on module 'lambda_function'",
  "errorType": "Runtime.HandlerNotFound",
  "stackTrace": []
}

Is there a reason this error would be thrown even though I clearly have that function written in this module? This is driving me nuts. Thanks for any help!

It should be:

def lambda_handler(event, context):

not

def lambda_helper(event, context):