diff --git a/libraries/AdaptiveExpressions/ExpressionFunctions.cs b/libraries/AdaptiveExpressions/ExpressionFunctions.cs
index 6453ebb43d..0e253c0e06 100644
--- a/libraries/AdaptiveExpressions/ExpressionFunctions.cs
+++ b/libraries/AdaptiveExpressions/ExpressionFunctions.cs
@@ -3118,21 +3118,36 @@ private static IDictionary<string, ExpressionEvaluator> GetStandardFunctions()
                 // String
                 new ExpressionEvaluator(
                     ExpressionType.Concat,
-                    Apply(
+                    ApplySequence(
                         args =>
                         {
-                            var builder = new StringBuilder();
-                            foreach (var arg in args)
+                            var firstItem = args[0];
+                            var secondItem = args[1];
+                            var isFirstList = TryParseList(firstItem, out var firstList);
+                            var isSecondList = TryParseList(secondItem, out var secondList);
+
+                            if (firstItem == null && secondItem == null)
                             {
-                                if (arg != null)
-                                {
-                                    builder.Append(arg.ToString());
-                                }
+                                return null;
+                            }
+                            else if (firstItem == null && isSecondList)
+                            {
+                                return secondList;
+                            }
+                            else if (secondItem == null && isFirstList)
+                            {
+                                return firstList;
+                            }
+                            else if (isFirstList && isSecondList)
+                            {
+                                return firstList.OfType<object>().Concat(secondList.OfType<object>()).ToList();
+                            }
+                            else
+                            {
+                                return $"{firstItem?.ToString()}{secondItem?.ToString()}";
                             }
-
-                            return builder.ToString();
                         }),
-                    ReturnType.String,
+                    ReturnType.Array | ReturnType.String,
                     ValidateAtLeastOne),
                 new ExpressionEvaluator(
                     ExpressionType.Length,
diff --git a/tests/AdaptiveExpressions.Tests/ExpressionParserTests.cs b/tests/AdaptiveExpressions.Tests/ExpressionParserTests.cs
index fe8edde863..2cbe65c333 100644
--- a/tests/AdaptiveExpressions.Tests/ExpressionParserTests.cs
+++ b/tests/AdaptiveExpressions.Tests/ExpressionParserTests.cs
@@ -813,6 +813,9 @@ public class ExpressionParserTests
             Test("contains(items, 'hi')", false),
             Test("contains(bag, 'three')", true),
             Test("contains(bag, 'xxx')", false),
+            Test("concat(null, [1, 2], null)", new List<object> { 1, 2 }),
+            Test("concat(createArray(1, 2), createArray(3, 4))", new List<object> { 1, 2, 3, 4 }),
+            Test("concat(['a', 'b'], ['b', 'c'], ['c', 'd'])", new List<object> { "a", "b", "b", "c", "c", "d" }),
             Test("count(split(hello,'e'))", 2),
             Test("count(createArray('h', 'e', 'l', 'l', 'o'))", 5),
             Test("empty('')", true),