diff --git a/pysteps/downscaling/rainfarm.py b/pysteps/downscaling/rainfarm.py
index d9906390d..b40e359df 100644
--- a/pysteps/downscaling/rainfarm.py
+++ b/pysteps/downscaling/rainfarm.py
@@ -157,7 +157,7 @@ def compute_psd(array, fft_size):
     )
 
     # Inverse FFT to obtain the merged array in the spatial domain
-    merged = np.real(np.fft.ifftn(fft_merged)) / len(fft_merged)
+    merged = np.real(np.fft.ifftn(fft_merged)) / fft_merged.size
 
     return merged