diff --git a/src/builtins/map.js b/src/builtins/map.js
index 578f0b6..a785d99 100644
--- a/src/builtins/map.js
+++ b/src/builtins/map.js
@@ -78,16 +78,19 @@ export default function map ( inputdir, outputdir, options ) {
 								return reject( err );
 							}
 
-							if ( result === null ) return fulfil();
+							return Promise.resolve( result )
+								.then( result => {
+									if ( result === null ) return fulfil();
 
-							const codepath = resolve( this.cachedir, filename );
+									const codepath = resolve( this.cachedir, filename );
 
-							const { code, map } = processResult( result, data, src, dest, codepath );
+									const { code, map } = processResult( result, data, src, dest, codepath );
 
-							writeToCacheDir( code, map, codepath, dest )
-								.then( () => symlinkOrCopy( codepath ).to( dest ) )
-								.then( () => options.cache[ filename ] = codepath )
-								.then( fulfil );
+									writeToCacheDir( code, map, codepath, dest )
+										.then( () => symlinkOrCopy( codepath ).to( dest ) )
+										.then( () => options.cache[ filename ] = codepath )
+										.then( fulfil );
+								});
 						})
 						.catch( reject );
 				}).catch( err => {
diff --git a/test/transform.js b/test/transform.js
index 3582cd7..98295f8 100644
--- a/test/transform.js
+++ b/test/transform.js
@@ -190,5 +190,19 @@ module.exports = function () {
 				assert.equal( read( 'tmp/foo/foo.md' ), read( 'tmp/output/foo.md' ) );
 			});
 		});
+
+		it( 'accepts promises from file transformers', function () {
+			return gobble( 'tmp/foo' ).transform( function ( input ) {
+				return new Promise( function ( fulfil ) {
+					process.nextTick( function () {
+						fulfil(input);
+					});
+				});
+			}).build({
+				dest: 'tmp/output'
+			}).then( function () {
+				assert.equal( read( 'tmp/foo/foo.md' ), read( 'tmp/output/foo.md' ) );
+			});
+		});
 	});
 };