diff --git a/lasp/tools/tools.py b/lasp/tools/tools.py
index fdf69a8..e9bbf15 100644
--- a/lasp/tools/tools.py
+++ b/lasp/tools/tools.py
@@ -136,7 +136,7 @@ def smoothSpectralData(freq, M, sw: SmoothingWidth,
     x0 = 1 if freq[0] == 0 else 0  # Skip first data point if zero frequency
     Psm[0] = P[0]                  # Reuse old value in case first data..
                                    # ..point is skipped. Not plotted any way.
-    P[-1] = P[-2]                  # Last data point (Nyquist f) often invalid
+    P[-1] = P[-2]                  # Last data point (Nyquist f) can be invalid
 
     # Loop through data points
     for x in range(x0, L):
@@ -165,6 +165,10 @@ def smoothSpectralData(freq, M, sw: SmoothingWidth,
         xl = bisect.bisect_left(freq, fl)  # index corresponding to fl
         xu = bisect.bisect_left(freq, fu)
 
+        # Guarantee window length of at least 1
+        if xu - xl <= 0:
+            xl = xu - 1
+
         # Calculate window
         g = np.zeros(xu-xl)
         for n, xi in enumerate(range(xl, xu)):