diff --git a/src/Statement.js b/src/Statement.js
index 8723ad5..c9b01f8 100644
--- a/src/Statement.js
+++ b/src/Statement.js
@@ -61,6 +61,7 @@ export default class Statement {
 
 				if ( node.type === 'ThisExpression' && contextDepth === 0 ) {
 					module.magicString.overwrite( node.start, node.end, 'undefined' );
+					module.bundle.onwarn( 'The `this` keyword is equivalent to `undefined` at the top level of an ES module, and has been rewritten' );
 				}
 
 				if ( node._scope ) scope = node._scope;
diff --git a/test/form/this-is-undefined/_config.js b/test/form/this-is-undefined/_config.js
index 155461a..7513402 100644
--- a/test/form/this-is-undefined/_config.js
+++ b/test/form/this-is-undefined/_config.js
@@ -1,3 +1,6 @@
 module.exports = {
-	description: 'top-level `this` expression is rewritten as `undefined`'
+	description: 'top-level `this` expression is rewritten as `undefined`',
+	options: {
+		onwarn: () => {}
+	}
 };
diff --git a/test/function/warn-on-top-level-this/_config.js b/test/function/warn-on-top-level-this/_config.js
new file mode 100644
index 0000000..c667044
--- /dev/null
+++ b/test/function/warn-on-top-level-this/_config.js
@@ -0,0 +1,13 @@
+const assert = require( 'assert' );
+
+module.exports = {
+	description: 'warns on top-level this (#770)',
+	warnings: warnings => {
+		assert.deepEqual( warnings, [
+			'The `this` keyword is equivalent to `undefined` at the top level of an ES module, and has been rewritten'
+		]);
+	},
+	runtimeError: err => {
+		assert.equal( err.message, `Cannot set property 'foo' of undefined` );
+	}
+};
diff --git a/test/function/warn-on-top-level-this/main.js b/test/function/warn-on-top-level-this/main.js
new file mode 100644
index 0000000..3853449
--- /dev/null
+++ b/test/function/warn-on-top-level-this/main.js
@@ -0,0 +1 @@
+this.foo = 'bar';
diff --git a/test/test.js b/test/test.js
index 8996b27..0ed678e 100644
--- a/test/test.js
+++ b/test/test.js
@@ -279,14 +279,14 @@ describe( 'rollup', function () {
 
 			if ( config.skipIfWindows && process.platform === 'win32' ) return;
 
-			var options = extend( {}, config.options, {
+			var options = extend( {}, {
 				entry: FORM + '/' + dir + '/main.js',
 				onwarn: msg => {
 					if ( /No name was provided for/.test( msg ) ) return;
 					if ( /as external dependency/.test( msg ) ) return;
 					console.error( msg );
 				}
-			});
+			}, config.options );
 
 			( config.skip ? describe.skip : config.solo ? describe.only : describe)( dir, function () {
 				PROFILES.forEach( function ( profile ) {