diff --git a/src/Bundle.js b/src/Bundle.js
index cf4085a..b79057f 100644
--- a/src/Bundle.js
+++ b/src/Bundle.js
@@ -1,3 +1,4 @@
+import { decode } from 'sourcemap-codec';
 import { Bundle as MagicStringBundle } from 'magic-string';
 import first from './utils/first.js';
 import { find } from './utils/array.js';
@@ -354,6 +355,9 @@ export default class Bundle {
 
 			if ( this.hasLoaders || find( this.plugins, plugin => plugin.transform || plugin.transformBundle ) ) {
 				map = magicString.generateMap( {} );
+				if ( typeof map.mappings === 'string' ) {
+					map.mappings = decode( map.mappings );
+				}
 				map = collapseSourcemaps( file, map, usedModules, bundleSourcemapChain, this.onwarn );
 			} else {
 				map = magicString.generateMap({ file, includeContent: true });
diff --git a/src/utils/collapseSourcemaps.js b/src/utils/collapseSourcemaps.js
index 173f714..7a504ba 100644
--- a/src/utils/collapseSourcemaps.js
+++ b/src/utils/collapseSourcemaps.js
@@ -1,4 +1,4 @@
-import { encode, decode } from 'sourcemap-codec';
+import { encode } from 'sourcemap-codec';
 import { dirname, relative, resolve } from './path.js';
 
 class Source {
@@ -17,7 +17,7 @@ class Link {
 	constructor ( map, sources ) {
 		this.sources = sources;
 		this.names = map.names;
-		this.mappings = decode( map.mappings );
+		this.mappings = map.mappings;
 	}
 
 	traceMappings () {
diff --git a/src/utils/transform.js b/src/utils/transform.js
index b8ba263..2f0c167 100644
--- a/src/utils/transform.js
+++ b/src/utils/transform.js
@@ -1,8 +1,14 @@
+import { decode } from 'sourcemap-codec';
+
 export default function transform ( source, id, plugins ) {
 	const sourceMapChain = [];
 
 	const originalSourceMap = typeof source.map === 'string' ? JSON.parse( source.map ) : source.map;
 
+	if ( originalSourceMap && typeof originalSourceMap.mappings === 'string' ) {
+		originalSourceMap.mappings = decode( originalSourceMap.mappings );
+	}
+
 	const originalCode = source.code;
 	let ast = source.ast;
 
@@ -25,6 +31,10 @@ export default function transform ( source, id, plugins ) {
 					result.map = JSON.parse( result.map );
 				}
 
+				if ( result.map && typeof result.map.mappings === 'string' ) {
+					result.map.mappings = decode( result.map.mappings );
+				}
+
 				sourceMapChain.push( result.map || { missing: true, plugin: plugin.name }); // lil' bit hacky but it works
 				ast = result.ast;
 
diff --git a/src/utils/transformBundle.js b/src/utils/transformBundle.js
index 400e994..027c7f0 100644
--- a/src/utils/transformBundle.js
+++ b/src/utils/transformBundle.js
@@ -1,3 +1,5 @@
+import { decode } from 'sourcemap-codec';
+
 export default function transformBundle ( code, plugins, sourceMapChain ) {
 	return plugins.reduce( ( code, plugin ) => {
 		if ( !plugin.transformBundle ) return code;
@@ -22,6 +24,10 @@ export default function transformBundle ( code, plugins, sourceMapChain ) {
 		}
 
 		const map = typeof result.map === 'string' ? JSON.parse( result.map ) : result.map;
+		if ( map && typeof map.mappings === 'string' ) {
+			map.mappings = decode( map.mappings );
+		}
+
 		sourceMapChain.push( map );
 
 		return result.code;