diff --git a/.gitignore b/.gitignore
index 64ce99175..6ac1022b7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -27,3 +27,4 @@
 
 *.log
 .DS_STORE
+.idea
diff --git a/generate/input/descriptor.json b/generate/input/descriptor.json
index 0d1e24413..6e636829e 100644
--- a/generate/input/descriptor.json
+++ b/generate/input/descriptor.json
@@ -489,10 +489,24 @@
           }
         },
         "git_diff_tree_to_workdir": {
-          "ignore": true
+          "args": {
+            "old_tree": {
+              "isOptional": true
+            },
+            "opts": {
+              "isOptional": true
+            }
+          }
         },
         "git_diff_tree_to_workdir_with_index": {
-          "ignore": true
+          "args": {
+            "old_tree": {
+              "isOptional": true
+            },
+            "opts": {
+              "isOptional": true
+            }
+          }
         }
       }
     },
diff --git a/lib/commit.js b/lib/commit.js
index 1bc92b8f2..305ec7dea 100644
--- a/lib/commit.js
+++ b/lib/commit.js
@@ -173,7 +173,7 @@ Commit.prototype.getDiff = function(callback) {
     var diffs = parents.map(function(parent) {
       return parent.getTree().then(function(parentTree) {
         return commit.getTree().then(function(thisTree) {
-          return parentTree.diff(thisTree);
+          return thisTree.diff(parentTree);
         });
       });
     });
diff --git a/test/tests/diff.js b/test/tests/diff.js
index 23800e6ff..ecb9001e6 100644
--- a/test/tests/diff.js
+++ b/test/tests/diff.js
@@ -1,24 +1,66 @@
 var assert = require("assert");
 var path = require("path");
+var promisify = require("promisify-node");
+var fse = promisify(require("fs-extra"));
+var Diff = require("../../lib/diff");
 
 describe("Diff", function() {
+  var Repository = require("../../lib/repository");
   var reposPath = path.resolve("test/repos/workdir/.git");
   var oid = "fce88902e66c72b5b93e75bdb5ae717038b221f6";
+  var diffFilename = "wddiff.txt";
+  var diffFilepath = path.join(
+    path.resolve("test/repos/workdir"),
+    diffFilename
+  );
 
-  var Repository = require("../../lib/repository");
-  var Diff = require("../../lib/diff");
-
-  before(function() {
+  before(function(done) {
     var test = this;
 
     return Repository.open(reposPath).then(function(repository) {
       test.repository = repository;
 
-      return repository.getCommit(oid).then(function(commit) {
-        test.commit = commit;
+      return repository.getBranchCommit("master").then(function(masterCommit) {
+
+        return masterCommit.getTree().then(function(tree) {
+          test.masterCommitTree = tree;
+
+          return repository.getCommit(oid).then(function(commit) {
+            test.commit = commit;
 
-        return commit.getDiff().then(function(diff) {
-          test.diff = diff;
+            return commit.getDiff().then(function(diff) {
+              test.diff = diff;
+
+              fse.writeFile(diffFilepath, "1 line\n2 line\n3 line\n\n4")
+              .then(function() {
+                return test.repository.openIndex();
+              })
+              .then(function(indexResult) {
+                test.index = indexResult;
+                return test.index.read(1);
+              })
+              .then(function() {
+                return test.index.addByPath(diffFilename);
+              })
+              .then(function() {
+                return test.index.write();
+              })
+              .then(function() {
+                return test.index.writeTree();
+              })
+              .then(function() {
+                Diff.treeToWorkdirWithIndex(
+                  test.repository,
+                  test.masterCommitTree,
+                  null
+                )
+                .then(function(workdirDiff) {
+                  test.workdirDiff = workdirDiff;
+                  done();
+                });
+              });
+            });
+          });
         });
       });
     });
@@ -40,17 +82,31 @@ describe("Diff", function() {
     assert.equal(lines[1].origin(), Diff.LINE.CONTEXT);
     assert.equal(lines[2].origin(), Diff.LINE.CONTEXT);
 
-    var oldContent = "\n__Before submitting a pull request, please ensure " +
+    var oldContent = "__Before submitting a pull request, please ensure " +
       "both unit tests and lint checks pass.__\n";
-    assert.equal(lines[2].content(), oldContent);
+    assert.equal(lines[3].content(), oldContent);
     assert.equal(lines[3].origin(), Diff.LINE.DELETION);
-    assert.equal(lines[4].contentLen(), 90);
+    assert.equal(lines[3].contentLen(), 90);
 
     var newContent = "__Before submitting a pull request, please ensure " +
       "both that you've added unit tests to cover your shiny new code, " +
       "and that all unit tests and lint checks pass.__\n";
-    assert.equal(lines[3].content(), newContent);
+    assert.equal(lines[4].content(), newContent);
     assert.equal(lines[4].origin(), Diff.LINE.ADDITION);
-    assert.equal(lines[3].contentLen(), 162);
+    assert.equal(lines[4].contentLen(), 162);
+  });
+
+  it("can diff the workdir with index", function() {
+    var patches = this.workdirDiff.patches();
+    assert.equal(patches.length, 1);
+
+    var hunks = patches[0].hunks();
+    assert.equal(hunks.length, 1);
+
+    var lines = hunks[0].lines();
+    assert.equal(
+      lines[0].content().substr(0, lines[0].contentLen()),
+      "1 line\n"
+    );
   });
 });