diff --git a/src/utils/transform.js b/src/utils/transform.js
index 761a217..5ddde28 100644
--- a/src/utils/transform.js
+++ b/src/utils/transform.js
@@ -25,6 +25,10 @@ export default function transform ( source, id, transformers ) {
 						map: null
 					};
 				}
+				// `result.map` can only be a string if `result` isn't
+				else if ( typeof result.map === 'string' ) {
+					result.map = JSON.parse( result.map );
+				}
 
 				sourceMapChain.push( result.map );
 				ast = result.ast;
diff --git a/test/function/handles-stringified-sourcemaps/_config.js b/test/function/handles-stringified-sourcemaps/_config.js
new file mode 100644
index 0000000..d09a3ce
--- /dev/null
+++ b/test/function/handles-stringified-sourcemaps/_config.js
@@ -0,0 +1,22 @@
+module.exports = {
+	description: 'handles transforms that return stringified source maps (#377)',
+
+	options: {
+		plugins: [
+			{
+				transform: function ( code ) {
+					return {
+						code: code,
+						// just stringify an otherwise acceptable source map
+						map: JSON.stringify({ mappings: '' })
+					};
+				}
+			}
+		]
+	},
+
+	// ensure source maps are generated
+	bundleOptions: {
+		sourceMap: true
+	}
+};
diff --git a/test/function/handles-stringified-sourcemaps/main.js b/test/function/handles-stringified-sourcemaps/main.js
new file mode 100644
index 0000000..7a4e8a7
--- /dev/null
+++ b/test/function/handles-stringified-sourcemaps/main.js
@@ -0,0 +1 @@
+export default 42;