diff --git a/src/CloudNative.CloudEvents/CloudEventAttributes.cs b/src/CloudNative.CloudEvents/CloudEventAttributes.cs
index 49a46fb..a140db6 100644
--- a/src/CloudNative.CloudEvents/CloudEventAttributes.cs
+++ b/src/CloudNative.CloudEvents/CloudEventAttributes.cs
@@ -211,12 +211,21 @@ namespace CloudNative.CloudEvents
         void ICollection<KeyValuePair<string, object>>.Add(KeyValuePair<string, object> item)
         {
             object value = item.Value;
+            // Note: can't throw ArgumentNullException as the null value is only part of the argument.
+            if (value is null)
+            {
+                throw new InvalidOperationException(Strings.ErrorCannotAddNullAttributeValue);
+            }
             ValidateAndNormalize(item.Key, ref value);
             dict.Add(item.Key, value);
         }
 
         void IDictionary<string, object>.Add(string key, object value)
         {
+            if (value is null)
+            {
+                throw new ArgumentNullException(nameof(value), Strings.ErrorCannotAddNullAttributeValue);
+            }
             ValidateAndNormalize(key, ref value);
             dict.Add(key, value);
         }
@@ -266,7 +275,7 @@ namespace CloudNative.CloudEvents
             return dict.TryGetValue(key, out value);
         }
 
-        internal virtual bool ValidateAndNormalize(string key, ref object value)
+        private bool ValidateAndNormalize(string key, ref object value)
         {
             if (key.Equals(TypeAttributeName(this.SpecVersion), StringComparison.InvariantCultureIgnoreCase))
             {
@@ -297,7 +306,7 @@ namespace CloudNative.CloudEvents
             }
             else if (key.Equals(TimeAttributeName(this.SpecVersion), StringComparison.InvariantCultureIgnoreCase))
             {
-                if (value is null || value is DateTime)
+                if (value is DateTime)
                 {
                     return true;
                 }
@@ -343,7 +352,7 @@ namespace CloudNative.CloudEvents
             }
             else if (key.Equals(DataSchemaAttributeName(this.SpecVersion), StringComparison.InvariantCultureIgnoreCase))
             {
-                if (value is null || value is Uri)
+                if (value is Uri)
                 {
                     return true;
                 }
@@ -361,7 +370,7 @@ namespace CloudNative.CloudEvents
             }
             else if (key.Equals(DataContentTypeAttributeName(this.SpecVersion), StringComparison.InvariantCultureIgnoreCase))
             {
-                if (value is null || value is ContentType)
+                if (value is ContentType)
                 {
                     return true;
                 }
diff --git a/src/CloudNative.CloudEvents/Strings.Designer.cs b/src/CloudNative.CloudEvents/Strings.Designer.cs
index a56c94d..87ad78e 100644
--- a/src/CloudNative.CloudEvents/Strings.Designer.cs
+++ b/src/CloudNative.CloudEvents/Strings.Designer.cs
@@ -60,6 +60,15 @@ namespace CloudNative.CloudEvents {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Null values cannot be added as attributes.
+        /// </summary>
+        internal static string ErrorCannotAddNullAttributeValue {
+            get {
+                return ResourceManager.GetString("ErrorCannotAddNullAttributeValue", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to The &apos;contenttype&apos; attribute value must be a content-type expression compliant with RFC2046.
         /// </summary>
diff --git a/src/CloudNative.CloudEvents/Strings.resx b/src/CloudNative.CloudEvents/Strings.resx
index 209b1e4..35946d5 100644
--- a/src/CloudNative.CloudEvents/Strings.resx
+++ b/src/CloudNative.CloudEvents/Strings.resx
@@ -117,6 +117,9 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
+  <data name="ErrorCannotAddNullAttributeValue" xml:space="preserve">
+    <value>Null values cannot be added as attributes</value>
+  </data>
   <data name="ErrorContentTypeIsNotRFC2046" xml:space="preserve">
     <value>The 'contenttype' attribute value must be a content-type expression compliant with RFC2046</value>
   </data>
diff --git a/test/CloudNative.CloudEvents.UnitTests/CloudEventAttributesTest.cs b/test/CloudNative.CloudEvents.UnitTests/CloudEventAttributesTest.cs
index e696a1b..c85e3fc 100644
--- a/test/CloudNative.CloudEvents.UnitTests/CloudEventAttributesTest.cs
+++ b/test/CloudNative.CloudEvents.UnitTests/CloudEventAttributesTest.cs
@@ -29,5 +29,22 @@ namespace CloudNative.CloudEvents.UnitTests
             string attributeName = CloudEventAttributes.SpecVersionAttributeName();
             Assert.Throws<InvalidOperationException>(() => attributes[attributeName] = null);
         }
+
+        [Fact]
+        public void Dictionary_Add_NullValue()
+        {
+            IDictionary<string, object> attributes = new CloudEventAttributes(CloudEventsSpecVersion.Default, emptyExtensions);
+            string attributeName = CloudEventAttributes.TypeAttributeName();
+            Assert.Throws<ArgumentNullException>(() => attributes.Add(attributeName, null));
+        }
+
+        [Fact]
+        public void Collection_Add_NullValue()
+        {
+            ICollection<KeyValuePair<string, object>> attributes = new CloudEventAttributes(CloudEventsSpecVersion.Default, emptyExtensions);
+            string attributeName = CloudEventAttributes.TypeAttributeName();
+            var pair = KeyValuePair.Create(attributeName, default(object));
+            Assert.Throws<InvalidOperationException>(() => attributes.Add(pair));
+        }
     }
 }