diff --git a/Duplicati/Library/Utility/FilterExpression.cs b/Duplicati/Library/Utility/FilterExpression.cs
index 7dd46155f..54a7bee3f 100644
--- a/Duplicati/Library/Utility/FilterExpression.cs
+++ b/Duplicati/Library/Utility/FilterExpression.cs
@@ -351,7 +351,7 @@ namespace Duplicati.Library.Utility
         /// <inheritdoc />
         public string GetFilterHash()
         {
-            var hash = MD5HashHelper.GetHash(m_filters.Select(x => x.Filter));
+            var hash = MD5HashHelper.GetHash(m_filters?.Select(x => x.Filter));
 			return Utility.ByteArrayAsHexString(hash);
         }
 
@@ -587,9 +587,9 @@ namespace Duplicati.Library.Utility
         /// <param name="second">Second.</param>
         public static FilterExpression Combine(FilterExpression first, FilterExpression second)
         {
-            if (first == null)
+            if (first == null || first.Empty)
                 return second;
-            if (second == null)
+            if (second == null || second.Empty)
                 return first;
 
             if (first.Result != second.Result)
diff --git a/Duplicati/Library/Utility/MD5HashHelper.cs b/Duplicati/Library/Utility/MD5HashHelper.cs
index 97644c6ac..a9fab75f7 100644
--- a/Duplicati/Library/Utility/MD5HashHelper.cs
+++ b/Duplicati/Library/Utility/MD5HashHelper.cs
@@ -50,10 +50,13 @@ namespace Duplicati.Library.Utility
         {
             using (var md5 = MD5.Create())
             {
-                foreach (var str in inputStrings)
+                if (inputStrings != null)
                 {
-                    var inputBuffer = Encoding.UTF8.GetBytes(str);
-                    md5.TransformBlock(inputBuffer, 0, inputBuffer.Length, inputBuffer, 0);
+                    foreach (var str in inputStrings)
+                    {
+                        var inputBuffer = Encoding.UTF8.GetBytes(str);
+                        md5.TransformBlock(inputBuffer, 0, inputBuffer.Length, inputBuffer, 0);
+                    }
                 }
  
                 md5.TransformFinalBlock(new byte[0], 0, 0);