diff --git a/com.unity.ml-agents.extensions/Runtime/Match3/AbstractBoard.cs b/com.unity.ml-agents.extensions/Runtime/Match3/AbstractBoard.cs
index 59d01e4790..365d99cc13 100644
--- a/com.unity.ml-agents.extensions/Runtime/Match3/AbstractBoard.cs
+++ b/com.unity.ml-agents.extensions/Runtime/Match3/AbstractBoard.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using UnityEngine;
 
@@ -64,22 +65,43 @@ public abstract class AbstractBoard : MonoBehaviour
         /// <returns></returns>
         public abstract bool MakeMove(Move m);
 
+        /// <summary>
+        /// Return the total number of moves possible for the board.
+        /// </summary>
+        /// <returns></returns>
+        public int NumMoves()
+        {
+            return Move.NumPotentialMoves(Rows, Columns);
+        }
+
+        /// <summary>
+        /// An optional callback for when the all moves are invalid. Ideally, the game state should
+        /// be changed before this happens, but this is a way to get notified if not.
+        /// </summary>
+        public Action OnNoValidMovesAction;
+
+        /// <summary>
+        /// Iterate through all Moves on the board.
+        /// </summary>
+        /// <returns></returns>
         public IEnumerable<Move> AllMoves()
         {
             var currentMove = Move.FromMoveIndex(0, Rows, Columns);
-            var numMoves = Move.NumPotentialMoves(Rows, Columns);
-            for (var i = 0; i < numMoves; i++)
+            for (var i = 0; i < NumMoves(); i++)
             {
                 yield return currentMove;
                 currentMove.Advance(Rows, Columns);
             }
         }
 
+        /// <summary>
+        /// Iterate through all valid Moves on the board.
+        /// </summary>
+        /// <returns></returns>
         public IEnumerable<Move> ValidMoves()
         {
             var currentMove = Move.FromMoveIndex(0, Rows, Columns);
-            var numMoves = Move.NumPotentialMoves(Rows, Columns);
-            for (var i = 0; i < numMoves; i++)
+            for (var i = 0; i < NumMoves(); i++)
             {
                 if (IsMoveValid(currentMove))
                 {
@@ -89,11 +111,14 @@ public IEnumerable<Move> ValidMoves()
             }
         }
 
+        /// <summary>
+        /// Iterate through all invalid Moves on the board.
+        /// </summary>
+        /// <returns></returns>
         public IEnumerable<Move> InvalidMoves()
         {
             var currentMove = Move.FromMoveIndex(0, Rows, Columns);
-            var numMoves = Move.NumPotentialMoves(Rows, Columns);
-            for (var i = 0; i < numMoves; i++)
+            for (var i = 0; i < NumMoves(); i++)
             {
                 if (!IsMoveValid(currentMove))
                 {
diff --git a/com.unity.ml-agents.extensions/Runtime/Match3/Match3Actuator.cs b/com.unity.ml-agents.extensions/Runtime/Match3/Match3Actuator.cs
index 4c4485f643..8ad9c10b65 100644
--- a/com.unity.ml-agents.extensions/Runtime/Match3/Match3Actuator.cs
+++ b/com.unity.ml-agents.extensions/Runtime/Match3/Match3Actuator.cs
@@ -75,8 +75,31 @@ public void ResetData()
 
         IEnumerable<int> InvalidMoveIndices()
         {
+            var numValidMoves = m_Board.NumMoves();
+
             foreach (var move in m_Board.InvalidMoves())
             {
+                numValidMoves--;
+                if (numValidMoves == 0)
+                {
+                    // If all the moves are invalid and we mask all the actions out, this will cause an assert
+                    // later on in IDiscreteActionMask. Instead, fire a callback to the user if they provided one,
+                    // (or log a warning if not) and leave the last action unmasked. This isn't great, but
+                    // an invalid move should be easier to handle than an exception..
+                    if (m_Board.OnNoValidMovesAction != null)
+                    {
+                        m_Board.OnNoValidMovesAction();
+                    }
+                    else
+                    {
+                        Debug.LogWarning(
+                            "No valid moves are available. The last action will be left unmasked, so " +
+                            "an invalid move will be passed to AbstractBoard.MakeMove()."
+                        );
+                    }
+                    // This means the last move won't be returned as an invalid index.
+                    yield break;
+                }
                 yield return move.MoveIndex;
             }
         }
diff --git a/com.unity.ml-agents.extensions/Tests/Editor/Match3/AbstractBoardTests.cs b/com.unity.ml-agents.extensions/Tests/Editor/Match3/AbstractBoardTests.cs
index f60ec55d8d..35bf4c405a 100644
--- a/com.unity.ml-agents.extensions/Tests/Editor/Match3/AbstractBoardTests.cs
+++ b/com.unity.ml-agents.extensions/Tests/Editor/Match3/AbstractBoardTests.cs
@@ -2,7 +2,6 @@
 using System.Collections.Generic;
 using UnityEngine;
 using NUnit.Framework;
-using Unity.MLAgents.Sensors;
 using Unity.MLAgents.Extensions.Match3;
 
 namespace Unity.MLAgents.Extensions.Tests.Match3
diff --git a/com.unity.ml-agents.extensions/Tests/Editor/Match3/Match3ActuatorTests.cs b/com.unity.ml-agents.extensions/Tests/Editor/Match3/Match3ActuatorTests.cs
new file mode 100644
index 0000000000..32d1acc214
--- /dev/null
+++ b/com.unity.ml-agents.extensions/Tests/Editor/Match3/Match3ActuatorTests.cs
@@ -0,0 +1,88 @@
+using NUnit.Framework;
+using Unity.MLAgents.Actuators;
+using Unity.MLAgents.Extensions.Match3;
+using UnityEngine;
+
+namespace Unity.MLAgents.Extensions.Tests.Match3
+{
+    internal class SimpleBoard : AbstractBoard
+    {
+        public int LastMoveIndex;
+        public bool MovesAreValid = true;
+
+        public bool CallbackCalled;
+
+        public override int GetCellType(int row, int col)
+        {
+            return 0;
+        }
+
+        public override int GetSpecialType(int row, int col)
+        {
+            return 0;
+        }
+
+        public override bool IsMoveValid(Move m)
+        {
+            return MovesAreValid;
+        }
+
+        public override bool MakeMove(Move m)
+        {
+            LastMoveIndex = m.MoveIndex;
+            return MovesAreValid;
+        }
+
+        public void Callback()
+        {
+            CallbackCalled = true;
+        }
+    }
+
+    public class Match3ActuatorTests
+    {
+        [SetUp]
+        public void SetUp()
+        {
+            if (Academy.IsInitialized)
+            {
+                Academy.Instance.Dispose();
+            }
+        }
+
+        [TestCase(true)]
+        [TestCase(false)]
+        public void TestValidMoves(bool movesAreValid)
+        {
+            // Check that a board with no valid moves doesn't raise an exception.
+            var gameObj = new GameObject();
+            var board = gameObj.AddComponent<SimpleBoard>();
+            var agent = gameObj.AddComponent<Agent>();
+            gameObj.AddComponent<Match3ActuatorComponent>();
+
+            board.Rows = 5;
+            board.Columns = 5;
+            board.NumCellTypes = 5;
+            board.NumSpecialTypes = 0;
+
+            board.MovesAreValid = movesAreValid;
+            board.OnNoValidMovesAction = board.Callback;
+            board.LastMoveIndex = -1;
+
+            agent.LazyInitialize();
+            agent.RequestDecision();
+            Academy.Instance.EnvironmentStep();
+
+            if (movesAreValid)
+            {
+                Assert.IsFalse(board.CallbackCalled);
+            }
+            else
+            {
+                Assert.IsTrue(board.CallbackCalled);
+            }
+            Assert.AreNotEqual(-1, board.LastMoveIndex);
+        }
+
+    }
+}
diff --git a/com.unity.ml-agents.extensions/Tests/Editor/Match3/Match3ActuatorTests.cs.meta b/com.unity.ml-agents.extensions/Tests/Editor/Match3/Match3ActuatorTests.cs.meta
new file mode 100644
index 0000000000..3731b4e758
--- /dev/null
+++ b/com.unity.ml-agents.extensions/Tests/Editor/Match3/Match3ActuatorTests.cs.meta
@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: 2edf24df24ac426085cb31a94d063683
+timeCreated: 1603392289
\ No newline at end of file
diff --git a/com.unity.ml-agents.extensions/Tests/Editor/Match3/MoveTests.cs b/com.unity.ml-agents.extensions/Tests/Editor/Match3/MoveTests.cs
index abd10bc858..dc94355794 100644
--- a/com.unity.ml-agents.extensions/Tests/Editor/Match3/MoveTests.cs
+++ b/com.unity.ml-agents.extensions/Tests/Editor/Match3/MoveTests.cs
@@ -1,8 +1,4 @@
-using System;
-using System.Collections.Generic;
-using UnityEngine;
 using NUnit.Framework;
-using Unity.MLAgents.Sensors;
 using Unity.MLAgents.Extensions.Match3;
 
 namespace Unity.MLAgents.Extensions.Tests.Match3