diff --git a/Octokit.Tests/Helpers/AssertEx.cs b/Octokit.Tests/Helpers/AssertEx.cs
index 7044aec2..ed388d23 100644
--- a/Octokit.Tests/Helpers/AssertEx.cs
+++ b/Octokit.Tests/Helpers/AssertEx.cs
@@ -58,5 +58,12 @@ namespace Octokit.Tests.Helpers
                 await Throws<ArgumentException>(async () => await action(argument));
             }
         }
+
+        public static void IsReadOnlyCollection<T>(object instance) where T : class
+        {
+            var collection = instance as ICollection<T>;
+            // The collection == null case is for .NET 4.0
+            Assert.True(instance is IReadOnlyCollection<T> && (collection == null || collection.IsReadOnly));
+        }
     }
 }
diff --git a/Octokit.Tests/Models/PunchCardTests.cs b/Octokit.Tests/Models/PunchCardTests.cs
index 05f6f19e..b83efb34 100644
--- a/Octokit.Tests/Models/PunchCardTests.cs
+++ b/Octokit.Tests/Models/PunchCardTests.cs
@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using Octokit.Response;
+using Octokit.Tests.Helpers;
 using Xunit;
 
 public class PunchCardTests
@@ -61,7 +62,7 @@ public class PunchCardTests
 
             var punchCard = new PunchCard(points);
 
-            Assert.Null(punchCard.PunchPoints as ICollection<PunchCardPoint>);
+            AssertEx.IsReadOnlyCollection<PunchCardPoint>(punchCard.PunchPoints);
         }
     }
 }
\ No newline at end of file
diff --git a/Octokit.Tests/Models/SearchCodeRequestTests.cs b/Octokit.Tests/Models/SearchCodeRequestTests.cs
index 7035c6e4..86587d57 100644
--- a/Octokit.Tests/Models/SearchCodeRequestTests.cs
+++ b/Octokit.Tests/Models/SearchCodeRequestTests.cs
@@ -1,5 +1,5 @@
-using System.Collections.Generic;
-using Octokit;
+using Octokit;
+using Octokit.Tests.Helpers;
 using Xunit;
 
 public class SearchCodeRequestTests
@@ -14,7 +14,7 @@ public class SearchCodeRequestTests
             var result = request.MergedQualifiers();
 
             // If I can cast this to a writeable collection, then that defeats the purpose of a read only.
-            Assert.Null(result as ICollection<string>);
+            AssertEx.IsReadOnlyCollection<string>(result);
         }
     }
 }
diff --git a/Octokit.Tests/Models/SearchIssuesRequestTests.cs b/Octokit.Tests/Models/SearchIssuesRequestTests.cs
index b26d343c..03a48514 100644
--- a/Octokit.Tests/Models/SearchIssuesRequestTests.cs
+++ b/Octokit.Tests/Models/SearchIssuesRequestTests.cs
@@ -1,5 +1,5 @@
-using System.Collections.Generic;
-using Octokit;
+using Octokit;
+using Octokit.Tests.Helpers;
 using Xunit;
 
 internal class SearchIssuesRequestTests
@@ -14,7 +14,7 @@ internal class SearchIssuesRequestTests
             var result = request.MergedQualifiers();
 
             // If I can cast this to a writeable collection, then that defeats the purpose of a read only.
-            Assert.Null(result as ICollection<string>);
+            AssertEx.IsReadOnlyCollection<string>(result);
         }
     }
 }
diff --git a/Octokit.Tests/Models/SearchUsersRequestTests.cs b/Octokit.Tests/Models/SearchUsersRequestTests.cs
index 7a6a3a39..110a3f60 100644
--- a/Octokit.Tests/Models/SearchUsersRequestTests.cs
+++ b/Octokit.Tests/Models/SearchUsersRequestTests.cs
@@ -1,5 +1,5 @@
-using System.Collections.Generic;
-using Octokit;
+using Octokit;
+using Octokit.Tests.Helpers;
 using Xunit;
 
 internal class SearchUsersRequestTests
@@ -14,7 +14,7 @@ internal class SearchUsersRequestTests
             var result = request.MergedQualifiers();
 
             // If I can cast this to a writeable collection, then that defeats the purpose of a read only.
-            Assert.Null(result as ICollection<string>);
+            AssertEx.IsReadOnlyCollection<string>(result);
         }
     }
 }