diff --git a/lib/rapi/baserlib.py b/lib/rapi/baserlib.py
index 020d7bcba06057c73c849c46bb09b3a055f08178..fb4c48e77a731191764393cfce89bc3d7d530c7e 100644
--- a/lib/rapi/baserlib.py
+++ b/lib/rapi/baserlib.py
@@ -38,6 +38,10 @@ from ganeti import opcodes
 from ganeti import errors
 
 
+# Dummy value to detect unchanged parameters
+_DEFAULT = object()
+
+
 def BuildUriList(ids, uri_format, uri_fields=("name", "uri")):
   """Builds a URI list as used by index resources.
 
@@ -213,6 +217,53 @@ def FeedbackFn(ts, log_type, log_msg): # pylint: disable-msg=W0613
   logging.info("%s: %s", log_type, log_msg)
 
 
+def CheckType(value, exptype, descr):
+  """Abort request if value type doesn't match expected type.
+
+  @param value: Value
+  @type exptype: type
+  @param exptype: Expected type
+  @type descr: string
+  @param descr: Description of value
+  @return: Value (allows inline usage)
+
+  """
+  if not isinstance(value, exptype):
+    raise http.HttpBadRequest("%s: Type is '%s', but '%s' is expected" %
+                              (descr, type(value).__name__, exptype.__name__))
+
+  return value
+
+
+def CheckParameter(data, name, default=_DEFAULT, exptype=_DEFAULT):
+  """Check and return the value for a given parameter.
+
+  If no default value was given and the parameter doesn't exist in the input
+  data, an error is raise.
+
+  @type data: dict
+  @param data: Dictionary containing input data
+  @type name: string
+  @param name: Parameter name
+  @param default: Default value (can be None)
+  @param exptype: Expected type (can be None)
+
+  """
+  try:
+    value = data[name]
+  except KeyError:
+    if default is not _DEFAULT:
+      return default
+
+    raise http.HttpBadRequest("Required parameter '%s' is missing" %
+                              name)
+
+  if exptype is _DEFAULT:
+    return value
+
+  return CheckType(value, exptype, "'%s' parameter" % name)
+
+
 class R_Generic(object):
   """Generic class for resources.
 
@@ -279,13 +330,10 @@ class R_Generic(object):
     @param name: the required parameter
 
     """
-    if name in self.req.request_body:
-      return self.req.request_body[name]
-    elif args:
-      return args[0]
-    else:
-      raise http.HttpBadRequest("Required parameter '%s' is missing" %
-                                name)
+    if args:
+      return CheckParameter(self.req.request_body, name, default=args[0])
+
+    return CheckParameter(self.req.request_body, name)
 
   def useLocking(self):
     """Check if the request specifies locking.