diff --git a/Claunia.RsrcFork/ChangeLog b/Claunia.RsrcFork/ChangeLog
index f4e2f44..04f0d7c 100644
--- a/Claunia.RsrcFork/ChangeLog
+++ b/Claunia.RsrcFork/ChangeLog
@@ -1,3 +1,9 @@
+2016-09-06  Natalia Portillo  <claunia@claunia.com>
+
+	* Resource.cs: Added method to check if an ID exists.
+
+	* ResourceFork.cs: Corrected check of offsets.
+
 2016-09-02  Natalia Portillo  <claunia@claunia.com>
 
 	* Claunia.RsrcFork.csproj: Downgrade framework requisites to
diff --git a/Claunia.RsrcFork/Resource.cs b/Claunia.RsrcFork/Resource.cs
index 3866ae4..f6b925a 100644
--- a/Claunia.RsrcFork/Resource.cs
+++ b/Claunia.RsrcFork/Resource.cs
@@ -162,6 +162,11 @@ namespace Claunia.RsrcFork
         {
             return ids.ToArray();
         }
+
+        public bool ContainsId(short id)
+        {
+            return ids.Contains(id);
+        }
     }
 }
 
diff --git a/Claunia.RsrcFork/ResourceFork.cs b/Claunia.RsrcFork/ResourceFork.cs
index 4464391..cfb46d0 100644
--- a/Claunia.RsrcFork/ResourceFork.cs
+++ b/Claunia.RsrcFork/ResourceFork.cs
@@ -79,7 +79,7 @@ namespace Claunia.RsrcFork
             rsrcStream.Read(tmp, 0, 4);
             header.resourceMapLen = BitConverter.ToInt32(tmp.Reverse().ToArray(), 0);
 
-            if(header.resourceDataOff <= 0 || header.resourceDataOff <= 0 || header.resourceDataLen <= 0 || header.resourceMapLen <= 0)
+            if(header.resourceDataOff <= 0 || header.resourceMapOff <= 0 || header.resourceDataLen <= 0 || header.resourceMapLen <= 0)
                 throw new InvalidCastException("Not a resource fork");
 
             if(header.resourceDataOff + header.resourceDataLen > rsrcStream.Length ||