diff --git a/mpu/aws.py b/mpu/aws.py
index 8a44607..41c420d 100644
--- a/mpu/aws.py
+++ b/mpu/aws.py
@@ -47,6 +47,14 @@ def s3_download(source, destination, profile_name=None):
     destination : str
     profile_name : str, optional
         AWS profile
+
+    Raises
+    ------
+    botocore.exceptions.NoCredentialsError
+        Botocore is not able to find your credentials. Either specify
+        profile_name or add the environment variables AWS_ACCESS_KEY_ID,
+        AWS_SECRET_ACCESS_KEY and AWS_SESSION_TOKEN.
+        See https://boto3.readthedocs.io/en/latest/guide/configuration.html
     """
     session = boto3.Session(profile_name=profile_name)
     s3 = session.resource('s3')