diff --git a/lib/preprocessor.js b/lib/preprocessor.js
index c7889cc60..c19db9d80 100644
--- a/lib/preprocessor.js
+++ b/lib/preprocessor.js
@@ -30,10 +30,10 @@ var isBinary = Object.create(null);
 
 // TODO(vojta): instantiate preprocessors at the start to show warnings immediately
 var createPreprocessor = function(config, basePath, injector) {
-  var patterns = Object.keys(config);
   var alreadyDisplayedWarnings = Object.create(null);
 
   return function(file, done) {
+    var patterns = Object.keys(config);
     var thisFileIsBinary = isBinary[path.extname(file.originalPath)];
     var preprocessors = [];
     var nextPreprocessor = function(error, content) {
diff --git a/test/unit/preprocessor.spec.coffee b/test/unit/preprocessor.spec.coffee
index ae8cedc94..8cda0d3aa 100644
--- a/test/unit/preprocessor.spec.coffee
+++ b/test/unit/preprocessor.spec.coffee
@@ -39,6 +39,26 @@ describe 'preprocessor', ->
       done()
 
 
+  it 'should check patterns after creation when invoked', (done) ->
+    fakePreprocessor = sinon.spy (content, file, done) ->
+      file.path = file.path + '-preprocessed'
+      done null, 'new-content'
+
+    injector = new di.Injector [{'preprocessor:fake': ['factory', -> fakePreprocessor]}]
+    config = {'**/*.txt': ['fake']}
+    pp = m.createPreprocessor config, null, injector
+
+    file = {originalPath: '/some/a.js', path: 'path'}
+
+    config['**/*.js'] = ['fake']
+
+    pp file, ->
+      expect(fakePreprocessor).to.have.been.called
+      expect(file.path).to.equal 'path-preprocessed'
+      expect(file.content).to.equal 'new-content'
+      done()
+
+
   it 'should ignore not matching file', (done) ->
     fakePreprocessor = sinon.spy (content, file, done) ->
       done null, ''