diff --git a/src/core/IronPython/Hosting/Python.cs b/src/core/IronPython/Hosting/Python.cs
index e91f3d622..8367450dd 100644
--- a/src/core/IronPython/Hosting/Python.cs
+++ b/src/core/IronPython/Hosting/Python.cs
@@ -115,7 +115,7 @@ public static class Python {
         /// Given a ScriptRuntime gets the ScriptEngine for IronPython.
         /// </summary>
         public static ScriptEngine/*!*/ GetEngine(ScriptRuntime/*!*/ runtime) {
-            return runtime.GetEngineByTypeName(typeof(PythonContext).AssemblyQualifiedName);
+            return runtime.GetEngineByTypeName(GetPythonAssemblyName());
         }
 
         /// <summary>
@@ -302,7 +302,7 @@ value is bool &&
         /// </summary>
         public static LanguageSetup/*!*/ CreateLanguageSetup(IDictionary<string, object>? options) {
             var setup = new LanguageSetup(
-                typeof(PythonContext).AssemblyQualifiedName,
+                GetPythonAssemblyName(),
                 PythonContext.IronPythonDisplayName,
                 PythonContext.IronPythonNames.Split(';'),
                 PythonContext.IronPythonFileExtensions.Split(';')
@@ -358,13 +358,18 @@ public static string[] GetModuleFilenames(this ScriptEngine engine) {
         #region Private helpers
 
         private static PythonService/*!*/ GetPythonService(ScriptEngine/*!*/ engine) {
-            return engine.GetService<PythonService>(engine);
+            return engine.GetService<PythonService>(engine)!;  // not null since PythonContext.GetService<PythonService> override will create/return the service if it doesn't exist
         }
 
         private static PythonContext/*!*/ GetPythonContext(ScriptEngine/*!*/ engine) {
             return (PythonContext)HostingHelpers.GetLanguageContext(engine);
         }
 
+        private static string GetPythonAssemblyName() {
+            return typeof(PythonContext).AssemblyQualifiedName
+                ?? throw new InvalidOperationException($"Could not get assembly qualified name for {nameof(PythonContext)}.");
+        }
+
         #endregion
     }
 }
diff --git a/src/core/IronPython/Hosting/PythonCommandLine.cs b/src/core/IronPython/Hosting/PythonCommandLine.cs
index 1c4b4c453..bc9bb0483 100644
--- a/src/core/IronPython/Hosting/PythonCommandLine.cs
+++ b/src/core/IronPython/Hosting/PythonCommandLine.cs
@@ -433,9 +433,8 @@ protected override int RunInteractive() {
 
         protected override string Prompt {
             get {
-                object value;
-                if (Engine.GetSysModule().TryGetVariable("ps1", out value)) {
-                    var context = ((PythonScopeExtension)Scope.GetExtension(Language.ContextId)).ModuleContext.GlobalContext;
+                if (Engine.GetSysModule().TryGetVariable("ps1", out object? value)) {
+                    var context = ((PythonScopeExtension)Scope.GetExtension(Language.ContextId)!).ModuleContext.GlobalContext;
 
                     return PythonOps.ToString(context, value);
                 }
@@ -446,9 +445,8 @@ protected override string Prompt {
 
         public override string PromptContinuation {
             get {
-                object value;
-                if (Engine.GetSysModule().TryGetVariable("ps2", out value)) {
-                    var context = ((PythonScopeExtension)Scope.GetExtension(Language.ContextId)).ModuleContext.GlobalContext;
+                if (Engine.GetSysModule().TryGetVariable("ps2", out object? value)) {
+                    var context = ((PythonScopeExtension)Scope.GetExtension(Language.ContextId)!).ModuleContext.GlobalContext;
 
                     return PythonOps.ToString(context, value);
                 }
@@ -544,7 +542,8 @@ private void RunStartup() {
 
             Action action = delegate () {
                 try {
-                    su.Compile(pco, ErrorSink).Run(Scope);
+                    var sc = su.Compile(pco, ErrorSink) ?? throw new InvalidOperationException("Compilation failed without reporting errors");
+                    sc.Run(Scope);
                 } catch (Exception e) {
                     if (e is SystemExitException) {
                         throw;
@@ -659,7 +658,7 @@ private int RunFileWorker(string/*!*/ fileName) {
             PythonModule module = PythonContext.CompileModule(
                 fileName,
                 "__main__",
-                PythonContext.CreateFileUnit(String.IsNullOrEmpty(fileName) ? null : fileName, PythonContext.DefaultEncoding),
+                PythonContext.CreateFileUnit(fileName, PythonContext.DefaultEncoding),
                 modOpt,
                 out compiledCode);
             PythonContext.PublishModule("__main__", module);
diff --git a/src/core/IronPython/Hosting/PythonService.cs b/src/core/IronPython/Hosting/PythonService.cs
index 4a0d39ccc..260b58107 100644
--- a/src/core/IronPython/Hosting/PythonService.cs
+++ b/src/core/IronPython/Hosting/PythonService.cs
@@ -94,7 +94,7 @@ public PythonService(PythonContext/*!*/ context, ScriptEngine/*!*/ engine) {
 
         public string[] GetModuleFilenames() {
             List<string> res = new List<string>();
-            if ((object)_engine.GetSysModule().GetVariable("modules") is PythonDictionary dict) {
+            if ((object?)_engine.GetSysModule().GetVariable("modules") is PythonDictionary dict) {
                 foreach (var kvp in dict) {
                     if (kvp.Key is string key && kvp.Value is PythonModule module) {
                         var modDict = module.Get__dict__();
@@ -130,7 +130,7 @@ public ObjectHandle GetLocalCommandDispatcher() {
             return new ObjectHandle((Action<Action>)(action => _context.DispatchCommand(action)));
         }
 
-        public override object InitializeLifetimeService() {
+        public override object? InitializeLifetimeService() {
             return _engine.InitializeLifetimeService();
         }
 #endif
diff --git a/src/core/IronPython/Runtime/CompiledLoader.cs b/src/core/IronPython/Runtime/CompiledLoader.cs
index cf802eff5..d7acaf2cd 100644
--- a/src/core/IronPython/Runtime/CompiledLoader.cs
+++ b/src/core/IronPython/Runtime/CompiledLoader.cs
@@ -21,7 +21,7 @@ internal void AddScriptCode(ScriptCode code) {
                 if (onDiskCode.ModuleName == "__main__") {
                     _codes["__main__"] = onDiskCode;
                 } else {
-                    string name = code.SourceUnit.Path;
+                    string name = code.SourceUnit.Path ?? throw new InvalidOperationException("OnDiskScriptCode must have a path");
                     name = name.Replace(Path.DirectorySeparatorChar, '.');
                     if (name.EndsWith("__init__.py", StringComparison.Ordinal)) {
                         name = name.Substring(0, name.Length - ".__init__.py".Length);
diff --git a/src/core/IronPython/Runtime/Operations/PythonOps.cs b/src/core/IronPython/Runtime/Operations/PythonOps.cs
index 9a88b06e7..e1e6f9a7b 100644
--- a/src/core/IronPython/Runtime/Operations/PythonOps.cs
+++ b/src/core/IronPython/Runtime/Operations/PythonOps.cs
@@ -3390,7 +3390,7 @@ public static bool IsPythonType(PythonType type) {
 
         public static object? PublishModule(CodeContext/*!*/ context, string name) {
             context.LanguageContext.SystemStateModules.TryGetValue(name, out object? original);
-            var module = ((PythonScopeExtension)context.GlobalScope.GetExtension(context.LanguageContext.ContextId)).Module;
+            var module = ((PythonScopeExtension)context.GlobalScope.GetExtension(context.LanguageContext.ContextId)!).Module;
             context.LanguageContext.SystemStateModules[name] = module;
             return original;
         }
@@ -3442,7 +3442,7 @@ public static void Warn(CodeContext/*!*/ context, PythonType category, string me
             }
         }
 
-        public static void ShowWarning(CodeContext/*!*/ context, PythonType category, string message, string filename, int lineNo) {
+        public static void ShowWarning(CodeContext/*!*/ context, PythonType category, string message, string? filename, int lineNo) {
             PythonContext pc = context.LanguageContext;
             object? warnings = pc.GetWarningsModule();
             object? warn = null;
@@ -4335,8 +4335,8 @@ internal static void ScopeSetMember(CodeContext/*!*/ context, Scope scope, strin
             PythonOps.SetAttr(context, scope, name, value);
         }
 
-        internal static object ScopeGetMember(CodeContext/*!*/ context, Scope scope, string name) {
-            if (((object)scope.Storage) is ScopeStorage scopeStorage) {
+        internal static object? ScopeGetMember(CodeContext/*!*/ context, Scope scope, string name) {
+            if (scope.Storage is ScopeStorage scopeStorage) {
                 return scopeStorage.GetValue(name, false);
             }
 
@@ -4344,7 +4344,7 @@ internal static object ScopeGetMember(CodeContext/*!*/ context, Scope scope, str
         }
 
         internal static bool ScopeTryGetMember(CodeContext/*!*/ context, Scope scope, string name, out object? value) {
-            if (((object)scope.Storage) is ScopeStorage scopeStorage) {
+            if (scope.Storage is ScopeStorage scopeStorage) {
                 return scopeStorage.TryGetValue(name, false, out value);
             }
 
@@ -4352,7 +4352,7 @@ internal static bool ScopeTryGetMember(CodeContext/*!*/ context, Scope scope, st
         }
 
         internal static bool ScopeContainsMember(CodeContext/*!*/ context, Scope scope, string name) {
-            if (((object)scope.Storage) is ScopeStorage scopeStorage) {
+            if (scope.Storage is ScopeStorage scopeStorage) {
                 return scopeStorage.HasValue(name, false);
             }
 
@@ -4360,7 +4360,7 @@ internal static bool ScopeContainsMember(CodeContext/*!*/ context, Scope scope,
         }
 
         internal static bool ScopeDeleteMember(CodeContext/*!*/ context, Scope scope, string name) {
-            if (((object)scope.Storage) is ScopeStorage scopeStorage) {
+            if (scope.Storage is ScopeStorage scopeStorage) {
                 return scopeStorage.DeleteValue(name, false);
             }
 
@@ -4370,7 +4370,7 @@ internal static bool ScopeDeleteMember(CodeContext/*!*/ context, Scope scope, st
         }
 
         internal static IList<object?> ScopeGetMemberNames(CodeContext/*!*/ context, Scope scope) {
-            if (((object)scope.Storage) is ScopeStorage scopeStorage) {
+            if (scope.Storage is ScopeStorage scopeStorage) {
                 List<object?> res = new List<object?>();
 
                 foreach (string name in scopeStorage.GetMemberNames()) {
diff --git a/src/dlr b/src/dlr
index a646d7e43..3b7c63816 160000
--- a/src/dlr
+++ b/src/dlr
@@ -1 +1 @@
-Subproject commit a646d7e43ac94a8ee3de2f8fcfa86a1cc7168563
+Subproject commit 3b7c63816005f842ae5e1156cf522cf42079b1e9