diff --git a/pynta/main.py b/pynta/main.py
index 3e8cf088..f7983362 100644
--- a/pynta/main.py
+++ b/pynta/main.py
@@ -123,7 +123,7 @@ def generate_slab(self,skip_launch=False):
         if self.a is None:
             a = get_lattice_parameters(self.metal,self.surface_type,self.software,self.lattice_opt_software_kwargs)
             print("computed lattice constants of: {} Angstroms".format(a))
-            if not isinstance(a,list):
+            if isinstance(a,float):
                 self.a = a
             else:
                 self.a = a[0]