diff --git a/src/main/scala/app/RepositoryViewerController.scala b/src/main/scala/app/RepositoryViewerController.scala
index 4ba43cb..1127d19 100644
--- a/src/main/scala/app/RepositoryViewerController.scala
+++ b/src/main/scala/app/RepositoryViewerController.scala
@@ -58,13 +58,14 @@
   get("/:owner/:repository/commits/:branch")(referrersOnly { repository =>
     val branchName = params("branch")
     val page       = params.getOrElse("page", "1").toInt
-
     JGitUtil.withGit(getRepositoryDir(repository.owner, repository.name)){ git =>
-      val (logs, hasNext) = JGitUtil.getCommitLog(git, branchName, page, 30)
-
-      repo.html.commits(Nil, branchName, repository, logs.splitWith{ (commit1, commit2) =>
-        view.helpers.date(commit1.time) == view.helpers.date(commit2.time)
-      }, page, hasNext)
+      JGitUtil.getCommitLog(git, branchName, page, 30) match {
+        case Right((logs, hasNext)) =>
+          repo.html.commits(Nil, branchName, repository, logs.splitWith{ (commit1, commit2) =>
+            view.helpers.date(commit1.time) == view.helpers.date(commit2.time)
+          }, page, hasNext)
+        case Left(_) => NotFound
+      }
     }
   })
   
@@ -77,12 +78,14 @@
     val page       = params.getOrElse("page", "1").toInt
 
     JGitUtil.withGit(getRepositoryDir(repository.owner, repository.name)){ git =>
-      val (logs, hasNext) = JGitUtil.getCommitLog(git, branchName, page, 30, path)
-
-      repo.html.commits(path.split("/").toList, branchName, repository,
-        logs.splitWith{ (commit1, commit2) =>
-          view.helpers.date(commit1.time) == view.helpers.date(commit2.time)
-        }, page, hasNext)
+      JGitUtil.getCommitLog(git, branchName, page, 30, path) match {
+        case Right((logs, hasNext)) =>
+          repo.html.commits(path.split("/").toList, branchName, repository,
+            logs.splitWith{ (commit1, commit2) =>
+              view.helpers.date(commit1.time) == view.helpers.date(commit2.time)
+            }, page, hasNext)
+        case Left(_) => NotFound
+      }
     }
   })
 
@@ -214,27 +217,25 @@
       repo.html.guide(repository)
     } else {
       JGitUtil.withGit(getRepositoryDir(repository.owner, repository.name)){ git =>
-        // get specified commit
-        val (revCommit, revision) = try {
-          val revision = if(revstr.isEmpty) repository.repository.defaultBranch else revstr
-          (JGitUtil.getRevCommitFromId(git, git.getRepository.resolve(revision)), revision)
-        } catch {
-          case e: NullPointerException => {
-            val revision = repository.branchList.head
-            (JGitUtil.getRevCommitFromId(git, git.getRepository.resolve(revision)), revision)
-          }
-        }
-        // get files
-        val files = JGitUtil.getFileList(git, revision, path)
-        // process README.md
-        val readme = files.find(_.name == "README.md").map { file =>
-          new String(JGitUtil.getContent(Git.open(getRepositoryDir(repository.owner, repository.name)), file.id, true).get, "UTF-8")
-        }
+        val revisions = Seq(if(revstr.isEmpty) repository.repository.defaultBranch else revstr, repository.branchList.head)
+        //val objectId = git.getRepository.resolve(revstr2)
 
-        repo.html.files(revision, repository,
-          if(path == ".") Nil else path.split("/").toList, // current path
-          new JGitUtil.CommitInfo(revCommit), // latest commit
-          files, readme)
+        // get specified commit
+        revisions.map { rev => (git.getRepository.resolve(rev), rev)}.find(_._1 != null).map { case (objectId, revision) =>
+          val revCommit = JGitUtil.getRevCommitFromId(git, objectId)
+
+          // get files
+          val files = JGitUtil.getFileList(git, revision, path)
+          // process README.md
+          val readme = files.find(_.name == "README.md").map { file =>
+            new String(JGitUtil.getContent(Git.open(getRepositoryDir(repository.owner, repository.name)), file.id, true).get, "UTF-8")
+          }
+
+          repo.html.files(revision, repository,
+            if(path == ".") Nil else path.split("/").toList, // current path
+            new JGitUtil.CommitInfo(revCommit), // latest commit
+            files, readme)
+        } getOrElse NotFound
       }
     }
   }
diff --git a/src/main/scala/app/WikiController.scala b/src/main/scala/app/WikiController.scala
index 8a4a22f..d746367 100644
--- a/src/main/scala/app/WikiController.scala
+++ b/src/main/scala/app/WikiController.scala
@@ -47,7 +47,10 @@
     val pageName = params("page")
 
     JGitUtil.withGit(getWikiRepositoryDir(repository.owner, repository.name)){ git =>
-      wiki.html.history(Some(pageName), JGitUtil.getCommitLog(git, "master", path = pageName + ".md")._1, repository)
+      JGitUtil.getCommitLog(git, "master", path = pageName + ".md") match {
+        case Right((logs, hasNext)) => wiki.html.history(Some(pageName), logs, repository)
+        case Left(_) => NotFound
+      }
     }
   })
   
@@ -117,7 +120,10 @@
   
   get("/:owner/:repository/wiki/_history")(referrersOnly { repository =>
     JGitUtil.withGit(getWikiRepositoryDir(repository.owner, repository.name)){ git =>
-      wiki.html.history(None, JGitUtil.getCommitLog(git, "master")._1, repository)
+      JGitUtil.getCommitLog(git, "master") match {
+        case Right((logs, hasNext)) => wiki.html.history(None, logs, repository)
+        case Left(_) => NotFound
+      }
     }
   })
 
diff --git a/src/main/scala/util/JGitUtil.scala b/src/main/scala/util/JGitUtil.scala
index 7866799..8d9b6eb 100644
--- a/src/main/scala/util/JGitUtil.scala
+++ b/src/main/scala/util/JGitUtil.scala
@@ -253,7 +253,7 @@
    * @param path filters by this path. default is no filter.
    * @return a tuple of the commit list and whether has next
    */
-  def getCommitLog(git: Git, revision: String, page: Int = 1, limit: Int = 0, path: String = ""): (List[CommitInfo], Boolean) = {
+  def getCommitLog(git: Git, revision: String, page: Int = 1, limit: Int = 0, path: String = ""): Either[String, (List[CommitInfo], Boolean)] = {
     val fixedPage = if(page <= 0) 1 else page
     
     @scala.annotation.tailrec
@@ -267,20 +267,25 @@
       }
     
     val revWalk = new RevWalk(git.getRepository)
-    revWalk.markStart(revWalk.parseCommit(git.getRepository.resolve(revision)))
-    if(path.nonEmpty){
-      revWalk.setRevFilter(new RevFilter(){
-        def include(walk: RevWalk, commit: RevCommit): Boolean = {
-          getDiffs(git, commit.getName, false).find(_.newPath == path).nonEmpty
-        }
-        override def clone(): RevFilter = this
-      })
+    val objectId = git.getRepository.resolve(revision)
+    if(objectId == null){
+      Left(s"${revision} can't be resolved.")
+    } else {
+      revWalk.markStart(revWalk.parseCommit(objectId))
+      if(path.nonEmpty){
+        revWalk.setRevFilter(new RevFilter(){
+          def include(walk: RevWalk, commit: RevCommit): Boolean = {
+            getDiffs(git, commit.getName, false).find(_.newPath == path).nonEmpty
+          }
+          override def clone(): RevFilter = this
+        })
+      }
+
+      val commits = getCommitLog(revWalk.iterator, 0, Nil)
+      revWalk.release
+
+      Right(commits)
     }
-    
-    val commits = getCommitLog(revWalk.iterator, 0, Nil)
-    revWalk.release
-    
-    commits
   }
   
   /**