diff --git a/website/_assets/js/tryFlow.js.es6.liquid b/website/_assets/js/tryFlow.js.es6.liquid
index 021727ef6af..56d3011b722 100644
--- a/website/_assets/js/tryFlow.js.es6.liquid
+++ b/website/_assets/js/tryFlow.js.es6.liquid
@@ -114,7 +114,10 @@ function getAnnotations(text, callback, options, editor) {
         CodeMirror.signal(editor, 'flowCoverage', coverage);
         return coverage.expressions.uncovered_locs
       })
-      .catch(() => []),
+      .catch(err => {
+        console.error(err)
+        return []
+      }),
     flow
       .then(flowProxy => flowProxy.checkContent('-', text))
       .then(errors => {
@@ -245,7 +248,10 @@ class AsyncLocalFlow {
   }
 
   coverage(filename, body) {
-    return Promise.resolve(this._flow.coverage(filename, body))
+    if (this._flow.coverage) {
+      return Promise.resolve(this._flow.coverage(filename, body))
+    }
+    return Promise.reject(Error('coverage method is missing'))
   }
 
   dumpTypes(filename, body) {
diff --git a/website/_assets/js/tryFlowWorker.js b/website/_assets/js/tryFlowWorker.js
index 80da7e7f171..9a9fdc07ae2 100644
--- a/website/_assets/js/tryFlowWorker.js
+++ b/website/_assets/js/tryFlowWorker.js
@@ -34,8 +34,12 @@ this.onmessage = function(e) {
       return;
     case "coverage":
       getFlow(data.version).then(function(flow) {
-        var result = flow.coverage(data.filename, data.body);
-        postMessage({id: data.id, type: "coverage", result: result});
+        if (flow.coverage) {
+          var result = flow.coverage(data.filename, data.body);
+          postMessage({id: data.id, type: "coverage", result: result});
+        } else {
+          postMessage({id: data.id, type: "coverage", err: Error('coverage method is missing')});
+        }
       })["catch"](function (e) {
         postMessage({id: data.id, type: "coverage", err: e});
       })