The peeled target isn't necessarily a Commit; it can be a Blob of any kind. Safe-cast it to Commit and only use it if it's not null. Should fix #721.

Author: asbjornu

src/GitVersionCore/VersionCalculation/BaseVersionCalculators/HighestTagBaseVersionStrategy.cs

@@ -30,7 +30,9 @@ public override IEnumerable<BaseVersion> GetVersions(GitVersionContext context)
                     SemanticVersion version;
                     if (SemanticVersion.TryParse(t.Name, context.Configuration.GitTagPrefix, out version))
                     {
-                        return new VersionTaggedCommit((Commit)t.PeeledTarget(), version, t.Name);
+                        var commit = t.PeeledTarget() as Commit;
+                        if (commit != null)
+                            return new VersionTaggedCommit(commit, version, t.Name);
                     }
                     return null;
                 })