diff --git a/src/perturbation.py b/src/perturbation.py
index b9a0f2717..ec9d87875 100644
--- a/src/perturbation.py
+++ b/src/perturbation.py
@@ -273,7 +273,7 @@ def plot_spectrum1D(pertx, perty, pertz):
 # data precision
 dtype = np.float64
 # ratio of solenoidal to compressive components
-if options.f_solenoidal is None or options.f_solenoidal is None:
+if (options.f_solenoidal is None) or (options.f_solenoidal=="None"):
     f_solenoidal = None
 else:
     f_solenoidal = min(max(float(options.f_solenoidal), 0.), 1.)