diff --git a/src/StreamFeed.cs b/src/StreamFeed.cs
index c675527..eab310a 100644
--- a/src/StreamFeed.cs
+++ b/src/StreamFeed.cs
@@ -131,6 +131,10 @@ public async Task<UpdateToTargetsResponse> UpdateActivityToTargetsAsync(ForeignI
             IEnumerable<string> newTargets = null,
             IEnumerable<string> removed = null)
         {
+            adds?.ForEach(FeedIdValidator.ThrowIfFeedIdIsInvalid);
+            newTargets?.ForEach(FeedIdValidator.ThrowIfFeedIdIsInvalid);
+            removed?.ForEach(FeedIdValidator.ThrowIfFeedIdIsInvalid);
+
             var payload = new
             {
                 foreign_id = foreignIdTime.ForeignId,
diff --git a/src/Utils/FeedIdValidator.cs b/src/Utils/FeedIdValidator.cs
new file mode 100644
index 0000000..e629e1a
--- /dev/null
+++ b/src/Utils/FeedIdValidator.cs
@@ -0,0 +1,61 @@
+using System;
+
+namespace Stream.Utils
+{
+    public static class FeedIdValidator
+    {
+        /// <summary>
+        /// Validates a fully qualified feed identifier. It should look like this: flat:myfeedname
+        /// We could use a Regex but that has a performance impact
+        /// so let's just iterate through the string and check for the correct format.
+        /// </summary>
+        public static void ThrowIfFeedIdIsInvalid(string feedId)
+        {
+            if (string.IsNullOrWhiteSpace(feedId))
+            {
+                throw new InvalidFeedIdException(feedId);
+            }
+
+            var foundColon = false;
+            var colonIndex = 0;
+            var index = 0;
+
+            foreach (var character in feedId)
+            {
+                if (character == ':')
+                {
+                    if (foundColon)
+                    {
+                        throw new InvalidFeedIdException(feedId);
+                    }
+
+                    if (index == 0 || index == feedId.Length - 1)
+                    {
+                        throw new InvalidFeedIdException(feedId);
+                    }
+
+                    foundColon = true;
+                    colonIndex = index;
+                }
+
+                index++;
+            }
+
+            if (!foundColon)
+            {
+                throw new InvalidFeedIdException(feedId);
+            }
+        }
+    }
+
+    /// <summary>
+    /// Exception thrown when a feed identifier is invalid.
+    /// The feed identifier should have a single colon. Example: flat:myfeedname
+    /// </summary>
+    public class InvalidFeedIdException : Exception
+    {
+        public InvalidFeedIdException(string feedId) : base($"Invalid feed id: {feedId}. It should look like this: flat:myfeedname")
+        {
+        }
+    }
+}
\ No newline at end of file
diff --git a/tests/FeedIdValidatorTests.cs b/tests/FeedIdValidatorTests.cs
new file mode 100644
index 0000000..ce5bbbe
--- /dev/null
+++ b/tests/FeedIdValidatorTests.cs
@@ -0,0 +1,22 @@
+using NUnit.Framework;
+using Stream.Utils;
+
+public class FeedIdValidatorTests
+{
+    [Test]
+    public void TestFeedIdValidator()
+    {
+        var invalidFeedIds = new[] { "nocolon", ":beginning", "ending:", "mul:tip:le:colons" };
+        var valifFeedIds = new[] { "flat:myfeedname" };
+
+        foreach (var feedId in invalidFeedIds)
+        {
+            Assert.Throws<InvalidFeedIdException>(() => FeedIdValidator.ThrowIfFeedIdIsInvalid(feedId));
+        }
+
+        foreach (var feedId in valifFeedIds)
+        {
+            Assert.DoesNotThrow(() => FeedIdValidator.ThrowIfFeedIdIsInvalid(feedId));
+        }
+    }
+}
\ No newline at end of file