diff --git a/client/core.lisp b/client/core.lisp
index 9c8b9a5..010fd5a 100644
--- a/client/core.lisp
+++ b/client/core.lisp
@@ -69,6 +69,7 @@
                      ((string-equal type "number") (push 'double-float type-list))
                      ((string-equal type "string") (push 'string type-list))
                      ((string-equal type "array") (push 'list type-list))
+                     ((string-equal type "null") (push 'null type-list))
                      (t
                       (error "Type ~S is not supported yet."
                              type)))))
@@ -418,6 +419,26 @@ lambda-list a separate defmethod."
       (jsonrpc/class:call client func-name arguments))))
 
 
+(eval-when (:compile-toplevel :load-toplevel :execute)
+  (defun %generate-client (class-name spec &key (export-symbols t))
+    (let* ((client-class (generate-client-class class-name spec :export-symbols export-symbols))
+           (object-classes
+             ;; The map from package::symbol to a code which defines
+             ;; a class for some complex object used as argument or
+             ;; result in an API:
+             (make-hash-table :test 'equal))
+           (methods (loop for method-spec in (gethash "methods" spec)
+                          appending (generate-method class-name method-spec object-classes
+                                                     :export-symbols export-symbols)))
+           (class-definitions
+             (loop for def being the hash-value of object-classes
+                   ;; Here each def contains a list of DEFCLASS + one or more methods.
+                   appending def)))
+      (values client-class
+              class-definitions
+              methods))))
+
+
 (defmacro generate-client (class-name url-or-path &key (export-symbols t))
   "Generates Common Lisp client by OpenRPC spec.
 
@@ -426,21 +447,10 @@ lambda-list a separate defmethod."
 
    URL-OR-PATH argument could be a string with HTTP URL of a spec, or a pathname
    if a spec should be read from the disc."
-  (let* ((spec (retrieve-spec (eval url-or-path)))
-         (client-class (generate-client-class class-name spec :export-symbols export-symbols))
-         (object-classes
-           ;; The map from package::symbol to a code which defines
-           ;; a class for some complex object used as argument or
-           ;; result in an API:
-           (make-hash-table :test 'equal))
-         (methods (loop for method-spec in (gethash "methods" spec)
-                        appending (generate-method class-name method-spec object-classes
-                                                   :export-symbols export-symbols)))
-         (class-definitions
-           (loop for def being the hash-value of object-classes
-                 ;; Here each def contains a list of DEFCLASS + one or more methods.
-                 appending def)))
-    `(eval-when (:compile-toplevel :load-toplevel :execute)
-       ,@client-class
-       ,@class-definitions
-       ,@methods)))
+  (let* ((spec (retrieve-spec (eval url-or-path))))
+    (multiple-value-bind (client-class class-definitions methods)
+        (%generate-client class-name spec :export-symbols export-symbols)
+      `(eval-when (:compile-toplevel :load-toplevel :execute)
+         ,@client-class
+         ,@class-definitions
+         ,@methods))))
diff --git a/openrpc-tests.asd b/openrpc-tests.asd
index 0f1a4d8..7fe5492 100644
--- a/openrpc-tests.asd
+++ b/openrpc-tests.asd
@@ -7,7 +7,8 @@
                "openrpc-tests/petshop"
                "openrpc-tests/server/interface"
                "openrpc-tests/client/deserialization"
-               "openrpc-tests/client/generation")
+               "openrpc-tests/client/generation"
+               "openrpc-tests/client/regression")
   :description "Test system for OPENRPC."
 
   :perform (test-op (op c)
diff --git a/qlfile.lock b/qlfile.lock
index 8b59e45..9bc696f 100644
--- a/qlfile.lock
+++ b/qlfile.lock
@@ -5,4 +5,4 @@
 ("ultralisp" .
  (:class qlot/source/dist:source-dist
   :initargs (:distribution "http://dist.ultralisp.org" :%version :latest)
-  :version "20230808001000"))
+  :version "20230911163002"))
diff --git a/t/client/regress-data/multiple-types/spec.json b/t/client/regress-data/multiple-types/spec.json
new file mode 100644
index 0000000..ffd1867
--- /dev/null
+++ b/t/client/regress-data/multiple-types/spec.json
@@ -0,0 +1,39 @@
+{
+  "methods": [
+    {
+      "name": "example",
+      "params": [
+        {
+          "name": "name",
+          "schema":
+          {
+            "type": ["string", "null"],
+            "maxLength": 255
+          },
+          "required": true,
+          "summary": "User name."
+        }
+      ],
+      "result": {
+        "name": "example_result",
+        "schema": {
+          "type": ["string", "null"],
+          "maxLength": 255
+        }
+      }, 
+      "summary": "Example method.",
+      "paramStructure": "by-name"
+    }
+  ],
+  "openrpc": "1.0.0",
+  "info": {
+    "title": "Example API",
+    "version": "0.1.0"
+  },
+  "servers": [
+    {
+      "name": "default",
+      "url": "https://example.org/"
+    }
+  ]
+}
diff --git a/t/client/regression.lisp b/t/client/regression.lisp
new file mode 100644
index 0000000..b81347f
--- /dev/null
+++ b/t/client/regression.lisp
@@ -0,0 +1,32 @@
+(uiop:define-package #:openrpc-tests/client/regression
+  (:use #:cl)
+  (:import-from #:rove
+                #:deftest)
+  (:import-from #:openrpc-client/core
+                #:%generate-client
+                #:retrieve-spec))
+(in-package #:openrpc-tests/client/regression)
+
+
+(deftest client-regression ()
+  (let ((spec
+          (rove:testing "Reading spec"
+            (retrieve-spec
+             ;; TODO: repeat this test for all subfolders of #P"t/client"
+             (asdf:system-relative-pathname :openrpc-tests
+                                            (make-pathname :directory '(:relative "t" "client" "regress-data" "multiple-types")
+                                                           :name "spec"
+                                                           :type "json"))))))
+    (rove:ok spec)
+    
+    (multiple-value-bind (client-class class-definitions methods)
+        (rove:testing "Generating client"
+          (%generate-client 'the-class spec :export-symbols nil))
+
+      ;; TODO: Here we need to compare results with forms read from:
+      ;; - #P"t/client/regress-data/multiple-types/client-class.lisp"
+      ;; - #P"t/client/regress-data/multiple-types/class-definitions.lisp"
+      ;; - #P"t/client/regress-data/multiple-types/methods.lisp"
+      (rove:ok client-class)
+      (rove:ok class-definitions)
+      (rove:ok methods))))