diff --git a/wsme/api.py b/wsme/api.py
index 9cadd28..da44f0f 100644
--- a/wsme/api.py
+++ b/wsme/api.py
@@ -1,5 +1,11 @@
+import traceback
 import functools
 import inspect
+import logging
+
+import wsme.exc
+
+log = logging.getLogger(__name__)
 
 
 def iswsmefunction(f):
@@ -130,3 +136,27 @@ class signature(object):
         return func
 
 sig = signature
+
+
+def format_exception(excinfo, debug=False):
+    """Extract informations that can be sent to the client."""
+    error = excinfo[1]
+    if isinstance(error, wsme.exc.ClientSideError):
+        r = dict(faultcode="Client",
+                 faultstring=error.faultstring)
+        log.warning("Client-side error: %s" % r['faultstring'])
+        r['debuginfo'] = None
+        return r
+    else:
+        faultstring = str(error)
+        debuginfo = "\n".join(traceback.format_exception(*excinfo))
+
+        log.error('Server-side error: "%s". Detail: \n%s' % (
+            faultstring, debuginfo))
+
+        r = dict(faultcode="Server", faultstring=faultstring)
+        if debug:
+            r['debuginfo'] = debuginfo
+        else:
+            r['debuginfo'] = None
+        return r
diff --git a/wsme/pecan.py b/wsme/pecan.py
index 3e70c54..7a478a8 100644
--- a/wsme/pecan.py
+++ b/wsme/pecan.py
@@ -1,3 +1,5 @@
+from __future__ import absolute_import
+
 import inspect
 import sys
 
@@ -6,7 +8,7 @@ import wsme.rest.args
 import wsme.rest.json
 import wsme.rest.xml
 
-pecan = sys.modules['pecan']
+import pecan
 
 
 class JSonRenderer(object):
@@ -14,6 +16,8 @@ class JSonRenderer(object):
         pass
 
     def render(self, template_path, namespace):
+        if 'faultcode' in namespace:
+            return wsme.rest.json.encode_error(None, namespace)
         return wsme.rest.json.encode_result(
             namespace['result'],
             namespace['datatype']
@@ -25,6 +29,8 @@ class XMLRenderer(object):
         pass
 
     def render(self, template_path, namespace):
+        if 'faultcode' in namespace:
+            return wsme.rest.xml.encode_error(None, namespace)
         return wsme.rest.xml.encode_result(
             namespace['result'],
             namespace['datatype']
@@ -52,11 +58,20 @@ def wsexpose(*args, **kwargs):
         funcdef.resolve_types(wsme.types.registry)
 
         def callfunction(self, *args, **kwargs):
-            args, kwargs = wsme.rest.args.get_args(
-                funcdef, args, kwargs,
-                pecan.request.body, pecan.request.content_type
-            )
-            result = f(self, *args, **kwargs)
+            try:
+                args, kwargs = wsme.rest.args.get_args(
+                    funcdef, args, kwargs,
+                    pecan.request.body, pecan.request.content_type
+                )
+                result = f(self, *args, **kwargs)
+            except:
+                data = wsme.api.format_exception(sys.exc_info())
+                if data['faultcode'] == 'Client':
+                    pecan.response.status = 400
+                else:
+                    pecan.response.status = 500
+                return data
+
             return dict(
                 datatype=funcdef.return_type,
                 result=result
diff --git a/wsme/root.py b/wsme/root.py
index e19d7c0..e7de08c 100644
--- a/wsme/root.py
+++ b/wsme/root.py
@@ -1,6 +1,5 @@
 import logging
 import sys
-import traceback
 import weakref
 
 from six import u, b
@@ -12,6 +11,7 @@ from wsme.exc import ClientSideError, MissingArgument, UnknownFunction
 from wsme.protocol import getprotocol
 from wsme.rest import scan_api
 from wsme import spore
+import wsme.api
 import wsme.types
 
 log = logging.getLogger(__name__)
@@ -200,7 +200,7 @@ class WSRoot(object):
 
         except Exception:
             e = sys.exc_info()[1]
-            infos = self._format_exception(sys.exc_info())
+            infos = wsme.api.format_exception(sys.exc_info(), self._debug)
             if isinstance(e, ClientSideError):
                 request.client_errorcount += 1
             else:
@@ -292,7 +292,7 @@ class WSRoot(object):
                 res.status = protocol.get_response_status(request)
                 res_content_type = protocol.get_response_contenttype(request)
         except Exception:
-            infos = self._format_exception(sys.exc_info())
+            infos = wsme.api.format_exception(sys.exc_info(), self._debug)
             request.server_errorcount += 1
             res.text = protocol.encode_error(context, infos)
             res.status = 500
@@ -325,29 +325,6 @@ class WSRoot(object):
                 return f, fdef, args
         raise UnknownFunction('/'.join(path))
 
-    def _format_exception(self, excinfo):
-        """Extract informations that can be sent to the client."""
-        error = excinfo[1]
-        if isinstance(error, ClientSideError):
-            r = dict(faultcode="Client",
-                     faultstring=error.faultstring)
-            log.warning("Client-side error: %s" % r['faultstring'])
-            r['debuginfo'] = None
-            return r
-        else:
-            faultstring = str(error)
-            debuginfo = "\n".join(traceback.format_exception(*excinfo))
-
-            log.error('Server-side error: "%s". Detail: \n%s' % (
-                faultstring, debuginfo))
-
-            r = dict(faultcode="Server", faultstring=faultstring)
-            if self._debug:
-                r['debuginfo'] = debuginfo
-            else:
-                r['debuginfo'] = None
-            return r
-
     def _html_format(self, content, content_types):
         try:
             from pygments import highlight