diff --git a/com.unity.ml-agents/CHANGELOG.md b/com.unity.ml-agents/CHANGELOG.md
index f907e98d7f..b231999948 100755
--- a/com.unity.ml-agents/CHANGELOG.md
+++ b/com.unity.ml-agents/CHANGELOG.md
@@ -25,6 +25,10 @@ and this project adheres to
 
 ### Bug Fixes
 #### com.unity.ml-agents (C#)
+- Fixed an issue where RayPerceptionSensor would raise an exception when the
+list of tags was empty, or a tag in the list was invalid (unknown, null, or
+empty string). (#4155)
+
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 
 ## [1.1.0-preview] - 2020-06-10
diff --git a/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs b/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs
index 418cb5825f..8996eb38d3 100644
--- a/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs
+++ b/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs
@@ -469,9 +469,24 @@ out DebugDisplayInfo.RayInfo debugRayOut
             if (castHit)
             {
                 // Find the index of the tag of the object that was hit.
-                for (var i = 0; i < input.DetectableTags.Count; i++)
+                var numTags = input.DetectableTags?.Count ?? 0;
+                for (var i = 0; i < numTags; i++)
                 {
-                    if (hitObject.CompareTag(input.DetectableTags[i]))
+                    var tagsEqual = false;
+                    try
+                    {
+                        var tag = input.DetectableTags[i];
+                        if (!string.IsNullOrEmpty(tag))
+                        {
+                            tagsEqual = hitObject.CompareTag(tag);
+                        }
+                    }
+                    catch (UnityException e)
+                    {
+                        // If the tag is null, empty, or not a valid tag, just ignore it.
+                    }
+
+                    if (tagsEqual)
                     {
                         rayOutput.HitTaggedObject = true;
                         rayOutput.HitTagIndex = i;
diff --git a/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensorComponentBase.cs b/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensorComponentBase.cs
index 5909eb84d0..094a36dfdc 100644
--- a/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensorComponentBase.cs
+++ b/com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensorComponentBase.cs
@@ -281,6 +281,11 @@ void OnDrawGizmosSelected()
             else
             {
                 var rayInput = GetRayPerceptionInput();
+                // We don't actually need the tags here, since they don't affect the display of the rays.
+                // Additionally, the user might be in the middle of typing the tag name when this is called,
+                // and there's no way to turn off the "Tag ... is not defined" error logs.
+                // So just don't use any tags here.
+                rayInput.DetectableTags = null;
                 for (var rayIndex = 0; rayIndex < rayInput.Angles.Count; rayIndex++)
                 {
                     DebugDisplayInfo.RayInfo debugRay;
diff --git a/com.unity.ml-agents/Tests/Editor/Sensor/RayPerceptionSensorTests.cs b/com.unity.ml-agents/Tests/Editor/Sensor/RayPerceptionSensorTests.cs
index 5f56fe321e..a15b25e790 100644
--- a/com.unity.ml-agents/Tests/Editor/Sensor/RayPerceptionSensorTests.cs
+++ b/com.unity.ml-agents/Tests/Editor/Sensor/RayPerceptionSensorTests.cs
@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using NUnit.Framework;
 using UnityEngine;
+using UnityEngine.TestTools;
 using Unity.MLAgents.Sensors;
 
 namespace Unity.MLAgents.Tests
@@ -336,6 +337,69 @@ public void TestStaticPerceive()
 
                 // Expected to hit the cube
                 Assert.AreEqual("cube", castOutput.RayOutputs[0].HitGameObject.name);
+                Assert.AreEqual(0, castOutput.RayOutputs[0].HitTagIndex);
+            }
+        }
+
+        [Test]
+        public void TestStaticPerceiveInvalidTags()
+        {
+            SetupScene();
+            var obj = new GameObject("agent");
+            var perception = obj.AddComponent<RayPerceptionSensorComponent3D>();
+
+            perception.RaysPerDirection = 0; // single ray
+            perception.MaxRayDegrees = 45;
+            perception.RayLength = 20;
+            perception.DetectableTags = new List<string>();
+            perception.DetectableTags.Add("Bad tag");
+            perception.DetectableTags.Add(null);
+            perception.DetectableTags.Add("");
+            perception.DetectableTags.Add(k_CubeTag);
+
+            var radii = new[] { 0f, .5f };
+            foreach (var castRadius in radii)
+            {
+                perception.SphereCastRadius = castRadius;
+                var castInput = perception.GetRayPerceptionInput();
+
+                // There's no clean way that I can find to check for a defined tag without
+                // logging an error.
+                LogAssert.Expect(LogType.Error, "Tag: Bad tag is not defined.");
+                var castOutput = RayPerceptionSensor.Perceive(castInput);
+
+                Assert.AreEqual(1, castOutput.RayOutputs.Length);
+
+                // Expected to hit the cube
+                Assert.AreEqual("cube", castOutput.RayOutputs[0].HitGameObject.name);
+                Assert.AreEqual(3, castOutput.RayOutputs[0].HitTagIndex);
+            }
+        }
+
+        [Test]
+        public void TestStaticPerceiveNoTags()
+        {
+            SetupScene();
+            var obj = new GameObject("agent");
+            var perception = obj.AddComponent<RayPerceptionSensorComponent3D>();
+
+            perception.RaysPerDirection = 0; // single ray
+            perception.MaxRayDegrees = 45;
+            perception.RayLength = 20;
+            perception.DetectableTags = null;
+
+            var radii = new[] { 0f, .5f };
+            foreach (var castRadius in radii)
+            {
+                perception.SphereCastRadius = castRadius;
+                var castInput = perception.GetRayPerceptionInput();
+                var castOutput = RayPerceptionSensor.Perceive(castInput);
+
+                Assert.AreEqual(1, castOutput.RayOutputs.Length);
+
+                // Expected to hit the cube
+                Assert.AreEqual("cube", castOutput.RayOutputs[0].HitGameObject.name);
+                Assert.AreEqual(-1, castOutput.RayOutputs[0].HitTagIndex);
             }
         }
     }