diff --git a/payload_generator/generate_delta_main.cc b/payload_generator/generate_delta_main.cc
index e9f09f7..a07df67 100644
--- a/payload_generator/generate_delta_main.cc
+++ b/payload_generator/generate_delta_main.cc
@@ -113,12 +113,6 @@
 
 namespace {
 
-bool IsDir(const char* path) {
-  struct stat stbuf;
-  TEST_AND_RETURN_FALSE_ERRNO(lstat(path, &stbuf) == 0);
-  return S_ISDIR(stbuf.st_mode);
-}
-
 void ParseSignatureSizes(const string& signature_sizes_flag,
                          vector<int>* signature_sizes) {
   signature_sizes->clear();
@@ -366,7 +360,8 @@
     LOG(INFO) << "Generating delta update";
     CHECK(!FLAGS_old_dir.empty());
     CHECK(!FLAGS_new_dir.empty());
-    if ((!IsDir(FLAGS_old_dir.c_str())) || (!IsDir(FLAGS_new_dir.c_str()))) {
+    if (!utils::IsDir(FLAGS_old_dir.c_str()) ||
+        !utils::IsDir(FLAGS_new_dir.c_str())) {
       LOG(FATAL) << "old_dir or new_dir not directory";
     }
   } else {
diff --git a/utils.cc b/utils.cc
index 6d2abac..70bcd34 100644
--- a/utils.cc
+++ b/utils.cc
@@ -520,6 +520,12 @@
   return lstat(path, &stbuf) == 0 && S_ISLNK(stbuf.st_mode) != 0;
 }
 
+bool IsDir(const char* path) {
+  struct stat stbuf;
+  TEST_AND_RETURN_FALSE_ERRNO(lstat(path, &stbuf) == 0);
+  return S_ISDIR(stbuf.st_mode);
+}
+
 std::string TempFilename(string path) {
   static const string suffix("XXXXXX");
   CHECK(StringHasSuffix(path, suffix));
diff --git a/utils.h b/utils.h
index 2961edf..e83db5c 100644
--- a/utils.h
+++ b/utils.h
@@ -105,6 +105,9 @@
 // Returns true if |path| exists and is a symbolic link.
 bool IsSymlink(const char* path);
 
+// Returns true if |path| exists and is a directory.
+bool IsDir(const char* path);
+
 // The last 6 chars of path must be XXXXXX. They will be randomly changed
 // and a non-existent path will be returned. Intentionally makes a copy
 // of the string passed in.
diff --git a/utils_unittest.cc b/utils_unittest.cc
index 77a7d9b..3103349 100644
--- a/utils_unittest.cc
+++ b/utils_unittest.cc
@@ -198,6 +198,20 @@
   EXPECT_TRUE(utils::RecursiveUnlinkDir(temp_dir));
 }
 
+TEST(UtilsTest, IsDirTest) {
+  string temp_dir;
+  EXPECT_TRUE(utils::MakeTempDirectory("isdir-test.XXXXXX", &temp_dir));
+  string temp_file = temp_dir + "temp-file";
+  EXPECT_TRUE(utils::WriteFile(temp_file.c_str(), "", 0));
+  string temp_symlink = temp_dir + "temp-symlink";
+  EXPECT_EQ(0, symlink(temp_dir.c_str(), temp_symlink.c_str()));
+  EXPECT_TRUE(utils::IsDir(temp_dir.c_str()));
+  EXPECT_FALSE(utils::IsDir(temp_file.c_str()));
+  EXPECT_FALSE(utils::IsDir(temp_symlink.c_str()));
+  EXPECT_FALSE(utils::IsDir("/non/existent/path"));
+  ASSERT_TRUE(utils::RecursiveUnlinkDir(temp_dir));
+}
+
 TEST(UtilsTest, TempFilenameTest) {
   const string original = "/foo.XXXXXX";
   const string result = utils::TempFilename(original);