diff --git a/pyFTS/data/artificial.py b/pyFTS/data/artificial.py
index 0f2916e..58feeef 100644
--- a/pyFTS/data/artificial.py
+++ b/pyFTS/data/artificial.py
@@ -163,8 +163,14 @@ def _append(additive, start, before, new):
     else:
         for k in range(start):
             new.insert(0,0)
-        tmp = np.array(before) + np.array(new)
-        return tmp
+
+        l = len(before)
+
+        if len(before) == 0:
+            tmp = np.array(new)
+        else:
+            tmp = np.array(before) + np.array(new[:l])
+        return tmp.tolist()
 
 
 
diff --git a/pyFTS/tests/general.py b/pyFTS/tests/general.py
index f8154e8..7eb404e 100644
--- a/pyFTS/tests/general.py
+++ b/pyFTS/tests/general.py
@@ -17,5 +17,8 @@ from pyFTS.common import Transformations, Membership
 from pyFTS.data import artificial
 
 cd = artificial.SignalEmulator()
-cd.stationary_gaussian(10,3,length=100)
-cd.incremental_gaussian(0.5,0,start=100,length=200)
+cd.stationary_gaussian(1,.2,length=10, it=1)
+cd.incremental_gaussian(0.5, 0,start=5,length=5)
+#cd.stationary_gaussian(3,.2,length=10, it=1, additive=True)
+print(len(cd.run()))
+