diff --git a/plone/app/discussion/browser/captcha.py b/plone/app/discussion/browser/captcha.py
index 3d3de3c..9a3c4f6 100644
--- a/plone/app/discussion/browser/captcha.py
+++ b/plone/app/discussion/browser/captcha.py
@@ -1,5 +1,6 @@
 from persistent import Persistent
 
+from z3c.form import validator
 from z3c.form.field import Fields
 
 from zope import interface, schema
@@ -20,15 +21,27 @@ from plone.app.discussion.browser.comments import CommentForm
 from plone.app.discussion.comment import Comment
 from plone.app.discussion.interfaces import IDiscussionSettings, ICaptcha
 
+HAS_CAPTCHA = False
 try:
     from plone.formwidget.captcha import CaptchaFieldWidget
+    from plone.formwidget.captcha.validator import CaptchaValidator
+    HAS_CAPTCHA = True
 except ImportError:
-    pass
+    # Fall back to SimpleFieldValidator, when Captcha is not installed,
+    # since otherwise the registration of the Captcha validator adapter
+    # would fail.
+    from z3c.form.validator import SimpleFieldValidator as CaptchaValidator
 
+HAS_RECAPTCHA = False
 try:
     from plone.formwidget.recaptcha import ReCaptchaFieldWidget
+    HAS_RECAPTCHA = True
 except ImportError:
-    pass
+    # Fall back to SimpleFieldValidator, when ReCaptcha is not installed,
+    # since otherwise the registration of the ReCaptcha validator adapter
+    # would fail.
+    from z3c.form.validator import SimpleFieldValidator as CaptchaValidator
+
 
 class Captcha(Persistent):
     interface.implements(ICaptcha)
@@ -37,31 +50,49 @@ class Captcha(Persistent):
 
 Captcha = factory(Captcha)
 
-class CaptchaExtender(extensible.FormExtender):
-    adapts(Interface, IDefaultBrowserLayer, CommentForm) # context, request, form
+if HAS_CAPTCHA or HAS_RECAPTCHA:
+    # Extend the comment form with captcha, only if a captcha solution is installed.
+    class CaptchaExtender(extensible.FormExtender):
+        adapts(Interface, IDefaultBrowserLayer, CommentForm) # context, request, form
 
-    fields = Fields(ICaptcha)
-    fields['captcha'].widgetFactory = CaptchaFieldWidget
+        fields = Fields(ICaptcha)
 
-    def __init__(self, context, request, form):
-        self.context = context
-        self.request = request
-        self.form = form
+        def __init__(self, context, request, form):
+            self.context = context
+            self.request = request
+            self.form = form
 
-        registry = queryUtility(IRegistry)
-        settings = registry.forInterface(IDiscussionSettings)
-        self.captcha = settings.captcha
+            registry = queryUtility(IRegistry)
+            settings = registry.forInterface(IDiscussionSettings)
+            self.captcha = settings.captcha
 
-    def update(self):
-        if self.captcha != 'disabled':
-            # Add all fields from the captcha interface
-            self.add(ICaptcha, prefix="")
-            if self.captcha == 'captcha':
-                self.form.fields['captcha'].widgetFactory = CaptchaFieldWidget
-            elif self.captcha == 'recaptcha':
-                self.form.fields['captcha'].widgetFactory = ReCaptchaFieldWidget
+        def update(self):
+            if self.captcha != 'disabled':
+                # Add captcha field if captcha is enabled in the registry
+                self.add(ICaptcha, prefix="")
+                if HAS_CAPTCHA and self.captcha == 'captcha':
+                    # If Captcha is installed and Captcha is enabled,
+                    # use the CaptchaFieldWidget
+                    self.form.fields['captcha'].widgetFactory = CaptchaFieldWidget
+                elif HAS_RECAPTCHA and self.captcha == 'recaptcha':
+                    # If ReCaptcha is installed and ReCaptcha is enabled,
+                    # use the ReCaptchaFieldWidget
+                    self.form.fields['captcha'].widgetFactory = ReCaptchaFieldWidget
+                else:
+                    #
+                    pass
 
-from z3c.form import validator
-from plone.formwidget.captcha.validator import CaptchaValidator
-# Register Captcha validator for the captcha field in the ICaptchaForm
-validator.WidgetValidatorDiscriminators(CaptchaValidator, field=ICaptcha['captcha'])
\ No newline at end of file
+    # Register Captcha validator for the captcha field in the ICaptchaForm
+    validator.WidgetValidatorDiscriminators(CaptchaValidator, field=ICaptcha['captcha'])
+
+else:
+    # This is necessary, otherwise the zcml registration of the CaptchaExtender
+    # would fail if no captcha solution is installed.
+    class CaptchaExtender(extensible.FormExtender):
+        adapts(Interface, IDefaultBrowserLayer, CommentForm)
+
+        def __init__(self, context, request, form):
+            pass
+
+        def update(self):
+            pass
\ No newline at end of file