diff --git a/src/Numerics/Distributions/Categorical.cs b/src/Numerics/Distributions/Categorical.cs
index 2bbf3f8f..6360fccf 100644
--- a/src/Numerics/Distributions/Categorical.cs
+++ b/src/Numerics/Distributions/Categorical.cs
@@ -237,7 +237,7 @@ namespace MathNet.Numerics.Distributions
         /// </summary>
         public double Entropy
         {
-            get { return _pmfNormalized.Sum(p => p*Math.Log(p)); }
+            get { return -_pmfNormalized.Sum(p => p*Math.Log(p)); }
         }
 
         /// <summary>