diff --git a/src/Bundle.js b/src/Bundle.js
index 27f4a04..dabbde2 100644
--- a/src/Bundle.js
+++ b/src/Bundle.js
@@ -13,6 +13,7 @@ import { unixizePath } from './utils/normalizePlatform.js';
 import transform from './utils/transform.js';
 import collapseSourcemaps from './utils/collapseSourcemaps.js';
 import callIfFunction from './utils/callIfFunction.js';
+import { isRelative } from './utils/path.js';
 
 export default class Bundle {
 	constructor ( options ) {
@@ -161,6 +162,7 @@ export default class Bundle {
 			return Promise.resolve( this.resolveId( source, module.id ) )
 				.then( resolvedId => {
 					if ( !resolvedId ) {
+						if ( isRelative( source ) ) throw new Error( `Could not resolve ${source} from ${module.id}` );
 						if ( !~this.external.indexOf( source ) ) this.onwarn( `Treating '${source}' as external dependency` );
 						module.resolvedIds[ source ] = source;
 
diff --git a/src/utils/path.js b/src/utils/path.js
index 5ef7f49..ec749a4 100644
--- a/src/utils/path.js
+++ b/src/utils/path.js
@@ -1,11 +1,16 @@
 // TODO does this all work on windows?
 
 export const absolutePath = /^(?:\/|(?:[A-Za-z]:)?[\\|\/])/;
+export const relativePath = /^\.?\.\//;
 
 export function isAbsolute ( path ) {
 	return absolutePath.test( path );
 }
 
+export function isRelative ( path ) {
+	return relativePath.test( path );
+}
+
 export function basename ( path ) {
 	return path.split( /(\/|\\)/ ).pop();
 }
diff --git a/test/function/no-relative-external/_config.js b/test/function/no-relative-external/_config.js
new file mode 100644
index 0000000..37f86df
--- /dev/null
+++ b/test/function/no-relative-external/_config.js
@@ -0,0 +1,8 @@
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'missing relative imports are an error, not a warning',
+	error: function ( err ) {
+		assert.ok( /Could not resolve \.\/missing\.js from/.test( err.message ) );
+	}
+};
diff --git a/test/function/no-relative-external/main.js b/test/function/no-relative-external/main.js
new file mode 100644
index 0000000..d0de2af
--- /dev/null
+++ b/test/function/no-relative-external/main.js
@@ -0,0 +1 @@
+import missing from './missing.js';