diff --git a/lasp/tools/tools.py b/lasp/tools/tools.py
index 47173bb..5f145a2 100644
--- a/lasp/tools/tools.py
+++ b/lasp/tools/tools.py
@@ -34,13 +34,14 @@ __all__ = ['SmoothingType', 'smoothSpectralData', 'SmoothingWidth']
 
 from enum import Enum, unique
 import bisect
+import copy
 import numpy as np
 
 
 @unique
 class SmoothingWidth(Enum):
     none = (0, 'No smoothing')
-    one = (1, '1/1stoctave smoothing')
+    one = (1, '1/1st octave smoothing')
     two = (2, '1/2th octave smoothing')
     three = (3, '1/3rd octave smoothing')
     six = (6, '1/6th octave smoothing')
@@ -105,7 +106,9 @@ def smoothSpectralData(freq, M, sw: SmoothingWidth,
     tr = 2  # truncate window after 2x std; shorter is faster and less accurate
 
     # Safety
+    MM = copy.deepcopy(M)
     Noct = sw.value[0]
+    assert M, "Smoothing function: input array is empty"
     assert Noct > 0, "'Noct' must be absolute positive"
     if Noct < 1:
         raise Warning('Check if \'Noct\' is entered correctly')
@@ -120,9 +123,9 @@ def smoothSpectralData(freq, M, sw: SmoothingWidth,
     L = M.shape[0]                   # number of data points
 
     if st == SmoothingType.levels:
-        P = 10**(M/10)  # magnitude [dB] --> power
+        P = 10**(MM/10)  # magnitude [dB] --> power
     else:
-        P = M  # data already given as power
+        P = MM  # data already given as power
     # TODO: This does not work due to complex numbers. Should be split up in
     # magnitude and phase.
     # elif st == SmoothingType.tf: