diff --git a/src/Bundle.js b/src/Bundle.js
index 27f4a04..2f9189b 100644
--- a/src/Bundle.js
+++ b/src/Bundle.js
@@ -161,6 +161,7 @@ export default class Bundle {
 			return Promise.resolve( this.resolveId( source, module.id ) )
 				.then( resolvedId => {
 					if ( !resolvedId ) {
+						if ( source[0] === '.' ) 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/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';