diff --git a/LibGit2Sharp/ObjectDatabase.cs b/LibGit2Sharp/ObjectDatabase.cs
index a79965613..05d2f5f6b 100644
--- a/LibGit2Sharp/ObjectDatabase.cs
+++ b/LibGit2Sharp/ObjectDatabase.cs
@@ -365,7 +365,9 @@ public virtual Commit CreateCommit(Signature author, Signature committer, string
 
             ObjectId commitId = Proxy.git_commit_create(repo.Handle, null, author, committer, message, tree, parentIds);
 
-            return repo.Lookup<Commit>(commitId);
+            Commit commit = repo.Lookup<Commit>(commitId);
+            Ensure.GitObjectIsNotNull(commit, commitId.Sha);
+            return commit;
         }
 
         /// <summary>
@@ -454,6 +456,8 @@ public virtual string ShortenObjectId(GitObject gitObject)
         /// <returns>A short string representation of the <see cref="ObjectId"/>.</returns>
         public virtual string ShortenObjectId(GitObject gitObject, int minLength)
         {
+            Ensure.ArgumentNotNull(gitObject, "gitObject");
+
             if (minLength <= 0 || minLength > ObjectId.HexSize)
             {
                 throw new ArgumentOutOfRangeException("minLength", minLength,
@@ -462,6 +466,11 @@ public virtual string ShortenObjectId(GitObject gitObject, int minLength)
 
             string shortSha = Proxy.git_object_short_id(repo.Handle, gitObject.Id);
 
+            if (shortSha == null)
+            {
+                throw new LibGit2SharpException("Unable to abbreviate SHA-1 value for GitObject " + gitObject.Id);
+            }
+
             if (minLength <= shortSha.Length)
             {
                 return shortSha;
diff --git a/LibGit2Sharp/Repository.cs b/LibGit2Sharp/Repository.cs
index 79c9077bc..fef0ccac1 100644
--- a/LibGit2Sharp/Repository.cs
+++ b/LibGit2Sharp/Repository.cs
@@ -476,7 +476,7 @@ internal GitObject LookupInternal(ObjectId id, GitObjectType type, FilePath know
 
             using (GitObjectSafeHandle obj = Proxy.git_object_lookup(handle, id, type))
             {
-                if (obj == null)
+                if (obj == null || obj.IsInvalid)
                 {
                     return null;
                 }