diff --git a/.babelrc b/.babelrc
index 50318ff..3cd9d25 100644
--- a/.babelrc
+++ b/.babelrc
@@ -13,7 +13,12 @@
         [ "es2015", { "loose": true } ]
       ],
       "plugins": [
-        "istanbul"
+        [ "istanbul", {
+          "exclude": [
+            "src/blob.js",
+            "test"
+          ]
+        } ]
       ]
     },
     "rollup": {
diff --git a/src/headers.js b/src/headers.js
index 1edf190..eb47f1c 100644
--- a/src/headers.js
+++ b/src/headers.js
@@ -245,6 +245,7 @@ function createHeadersIterator(target, kind) {
 
 const HeadersIteratorPrototype = Object.setPrototypeOf({
 	next() {
+		// istanbul ignore if
 		if (!this ||
 			Object.getPrototypeOf(this) !== HeadersIteratorPrototype) {
 			throw new TypeError('Value of `this` is not a HeadersIterator');