diff --git a/tiramisu/autolib.py b/tiramisu/autolib.py
index 54d165b..082afb0 100644
--- a/tiramisu/autolib.py
+++ b/tiramisu/autolib.py
@@ -211,7 +211,7 @@ def carry_out_calculation(option, context, callback, callback_params,
                     args.append(val)
                 else:
                     kwargs[key] = val
-        return calculate(callback, args, kwargs)
+        return calculate(callback, args, kwargs, returns_raise)
     else:
         # no value is multi
         # return a single value
@@ -224,7 +224,7 @@ def carry_out_calculation(option, context, callback, callback_params,
                     args.append(couple[0])
                 else:
                     kwargs[key] = couple[0]
-        ret = calculate(callback, args, kwargs)
+        ret = calculate(callback, args, kwargs, returns_raise)
         if not option.impl_is_optiondescription() and callback_params != {} and isinstance(ret, list) and \
                 option.impl_is_master_slaves('slave'):
             raise SlaveError(_("callback cannot return a list for a "
@@ -232,7 +232,7 @@ def carry_out_calculation(option, context, callback, callback_params,
         return ret
 
 
-def calculate(callback, args, kwargs):
+def calculate(callback, args, kwargs, returns_raise):
     """wrapper that launches the 'callback'
 
     :param callback: callback function
@@ -240,4 +240,10 @@ def calculate(callback, args, kwargs):
     :param kwargs: in the callback's arity, the named parameters
 
     """
-    return callback(*args, **kwargs)
+    if returns_raise:
+        try:
+            return callback(*args, **kwargs)
+        except ValueError, err:
+            return err
+    else:
+        return callback(*args, **kwargs)