diff --git a/.gitignore b/.gitignore
index 26a8dee..53be168 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,6 +16,7 @@ eggs/
 .eggs/
 lib/
 lib64/
+node_modules/
 parts/
 sdist/
 var/
diff --git a/.travis.yml b/.travis.yml
index 5106ebf..42a63ef 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,7 @@
 language: python
 
 python:
+  - 3.8
   - 3.7
   - 3.6
 
@@ -12,6 +13,7 @@ before_install:
 
 script:
   - while ! nc -z 127.0.0.1 9200; do sleep 1; done;
+  # - npm i vega@5.25.0
   - pip install --upgrade pip
   - pip install -r requirements.txt
   - pip install -r requirements-dev.txt
diff --git a/pybana/translators/vega/metrics.py b/pybana/translators/vega/metrics.py
index 3036916..0f7b2a4 100644
--- a/pybana/translators/vega/metrics.py
+++ b/pybana/translators/vega/metrics.py
@@ -21,7 +21,8 @@ def contribute(self, agg, bucket, response):
             if bucket and "doc_count" in bucket
             else response["hits"]["total"]
         )
-        bucket[agg["id"]] = {"value": ret}
+        if bucket is not None:
+            bucket[agg["id"]] = {"value": ret}
         return ret
 
 
diff --git a/requirements-dev.txt b/requirements-dev.txt
index a632617..643add5 100644
--- a/requirements-dev.txt
+++ b/requirements-dev.txt
@@ -1,3 +1,4 @@
+-r requirements.txt
 black==18.6b4
 coverage==4.5.3
 flake8==3.7.7
@@ -7,3 +8,4 @@ recommonmark==0.6.0
 sphinx==1.8.5
 twine==3.0.0
 wheel>=0.31.0
+pytest-html==1.22.1
diff --git a/requirements.txt b/requirements.txt
index aff8fee..1617927 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,3 +3,4 @@ elasticsearch-dsl==6.4.0
 hjson==3.0.1
 pendulum==2.1.2
 pytz>=2019.1
+pynumeral==0.1.2