Index: wflow-py/wflow/wflow_lib.py
===================================================================
diff -u -rbda446ad8d5ec510a514a29575822e732ddc217d -r911009b83b9c498347ba719f6a3bbec960014691
--- wflow-py/wflow/wflow_lib.py	(.../wflow_lib.py)	(revision bda446ad8d5ec510a514a29575822e732ddc217d)
+++ wflow-py/wflow/wflow_lib.py	(.../wflow_lib.py)	(revision 911009b83b9c498347ba719f6a3bbec960014691)
@@ -784,7 +784,10 @@
     Output:
         - result
     """
-    s = 1.0/(b + exp(-c * (X-a)))
+    try:
+        s = 1.0/(b + exp(-c * (X-a)))
+    except:
+        s = 1.0 / (b + np.exp(-c * (X - a)))
     return s
 
 def sCurveSlope(X,a=0.0,b=1.0,c=1.0):