diff --git a/source/openapi_client/schemas.d b/source/openapi_client/schemas.d
index a7e696b..fda0599 100644
--- a/source/openapi_client/schemas.d
+++ b/source/openapi_client/schemas.d
@@ -11,6 +11,7 @@ import vibe.core.log : logDebug;
 
 import std.container.rbtree : RedBlackTree;
 import std.file : mkdir, mkdirRecurse, write;
+import std.exception : enforce;
 import std.array : array, appender, split, Appender;
 import std.algorithm : canFind, skipOver;
 import std.path : buildNormalizedPath, dirName;
@@ -55,30 +56,30 @@ JsonSchema[string] jsonSchemaByRef;
  * name is in CamelCase.
  */
 string getClassNameFromSchemaName(string schemaName) {
-  static immutable RedBlackTree!string RESERVED_CLASSES = new RedBlackTree!string([
-        "cpp_type_info_ptr",
-        "Error",
-        "Exception",
-        "Object",
-        "Throwable",
-        "TypeInfo",
-        "TypeInfo_Array",
-        "TypeInfo_AssociativeArray",
-        "TypeInfo_Class",
-        "TypeInfo_Const",
-        "TypeInfo_Delegate",
-        "TypeInfo_Enum",
-        "TypeInfo_Function",
-        "TypeInfo_Interface",
-        "TypeInfo_Invariant",
-        "TypeInfo_Pointer",
-        "TypeInfo_Shared",
-        "TypeInfo_StaticArray",
-        "TypeInfo_Struct",
-        "TypeInfo_Tuple",
-        "TypeInfo_Vector",
-        "TypeInfo_Wild"
-      ]);
+  static bool[string] RESERVED_CLASSES = [
+        "cpp_type_info_ptr": true,
+        "Error": true,
+        "Exception": true,
+        "Object": true,
+        "Throwable": true,
+        "TypeInfo": true,
+        "TypeInfo_Array": true,
+        "TypeInfo_AssociativeArray": true,
+        "TypeInfo_Class": true,
+        "TypeInfo_Const": true,
+        "TypeInfo_Delegate": true,
+        "TypeInfo_Enum": true,
+        "TypeInfo_Function": true,
+        "TypeInfo_Interface": true,
+        "TypeInfo_Invariant": true,
+        "TypeInfo_Pointer": true,
+        "TypeInfo_Shared": true,
+        "TypeInfo_StaticArray": true,
+        "TypeInfo_Struct": true,
+        "TypeInfo_Tuple": true,
+        "TypeInfo_Vector": true,
+        "TypeInfo_Wild": true,
+      ];
   string className = toUpperCamelCase(tr(schemaName, ".", "_"));
   if (className in RESERVED_CLASSES)
     return className ~ "_";
@@ -117,6 +118,9 @@ string getSchemaNameFromRef(string ref_) {
  * files can be known as "model" or "dto" files.
  */
 void writeSchemaFiles(OasDocument oasDocument, string targetDir, string packageRoot) {
+  enforce(oasDocument !is null, "oasDocument must not be null");
+  enforce(oasDocument.components !is null, "oasDocument.components must not be null");
+  enforce(oasDocument.components.schemas !is null, "oasDocument.components.schemas must not be null");
   foreach (string schemaName, OasSchema schema; oasDocument.components.schemas) {
     JsonSchema jsonSchema = new JsonSchema();
     jsonSchema.schemaName = schemaName;
@@ -134,71 +138,71 @@ void writeSchemaFiles(OasDocument oasDocument, string targetDir, string packageR
 /**
  * A collection of variable names that cannot be used to generate Dlang code.
  */
-static immutable RedBlackTree!string RESERVED_WORDS = new RedBlackTree!string([
-      "abstract",
-      "alias",
-      "align",
-      "asm",
-      "assert",
-      "auto",
-      "bool",
-      "break",
-      "byte",
-      "case",
-      "catch",
-      "cast",
-      "char",
-      "class",
-      "const",
-      "continue",
-      "dchar",
-      "debug",
-      "default",
-      "delegate",
-      "double",
-      "dstring",
-      "else",
-      "enum",
-      "export",
-      "extern",
-      "finally",
-      "float",
-      "for",
-      "foreach",
-      "foreach_reverse",
-      "function",
-      "if",
-      "in",
-      "invariant",
-      "immutable",
-      "import",
-      "int",
-      "lazy",
-      "long",
-      "mixin",
-      "module",
-      "new",
-      "nothrow",
-      "package",
-      "private",
-      "protected",
-      "public",
-      "pure",
-      "real",
-      "ref",
-      "scope",
-      "short",
-      "string",
-      "struct",
-      "switch",
-      "ulong",
-      "union",
-      "version",
-      "wchar",
-      "while",
-      "with",
-      "wstring",
-    ]);
+bool[string] RESERVED_WORDS = [
+      "abstract": true,
+      "alias": true,
+      "align": true,
+      "asm": true,
+      "assert": true,
+      "auto": true,
+      "bool": true,
+      "break": true,
+      "byte": true,
+      "case": true,
+      "catch": true,
+      "cast": true,
+      "char": true,
+      "class": true,
+      "const": true,
+      "continue": true,
+      "dchar": true,
+      "debug": true,
+      "default": true,
+      "delegate": true,
+      "double": true,
+      "dstring": true,
+      "else": true,
+      "enum": true,
+      "export": true,
+      "extern": true,
+      "finally": true,
+      "float": true,
+      "for": true,
+      "foreach": true,
+      "foreach_reverse": true,
+      "function": true,
+      "if": true,
+      "in": true,
+      "invariant": true,
+      "immutable": true,
+      "import": true,
+      "int": true,
+      "lazy": true,
+      "long": true,
+      "mixin": true,
+      "module": true,
+      "new": true,
+      "nothrow": true,
+      "package": true,
+      "private": true,
+      "protected": true,
+      "public": true,
+      "pure": true,
+      "real": true,
+      "ref": true,
+      "scope": true,
+      "short": true,
+      "string": true,
+      "struct": true,
+      "switch": true,
+      "ulong": true,
+      "union": true,
+      "version": true,
+      "wchar": true,
+      "while": true,
+      "with": true,
+      "wstring": true,
+    ];
 
 /**
  * Writes a Dlang source file for a module that contains a class representing an OpenAPI