diff --git a/lib/python/fidasim/utils.py b/lib/python/fidasim/utils.py
index 970ea8516..ffe8d25a2 100644
--- a/lib/python/fidasim/utils.py
+++ b/lib/python/fidasim/utils.py
@@ -877,7 +877,7 @@ def extract_transp_plasma(filename, intime, grid, flux,
 
     max_flux = max(abs(rho))
 
-    mask = np.zeros(s,dtype='int')
+    mask = np.zeros(dims,dtype='int')
     w = np.where(flux <= max_flux) #where we have profiles
     mask[w] = 1