diff --git a/GENetLib/basis_fd.py b/GENetLib/basis_fd.py
index 74a0604..501a972 100644
--- a/GENetLib/basis_fd.py
+++ b/GENetLib/basis_fd.py
@@ -39,9 +39,9 @@ def basis_fd(btype=None, rangeval=None, nbasis=None, params=None, dropind=None,
         btype = "power"
     else:
         btype = "unknown"
-    if nbasis <= 0:
+    if nbasis is not None and nbasis<= 0:
         raise ValueError("Argument nbasis is not positive.")
-    if round(nbasis) != nbasis:
+    if nbasis is not None and round(nbasis) != nbasis:
         raise ValueError("Argument nbasis is not an integer.")
     if quadvals is None:
         quadvals = []
@@ -141,4 +141,4 @@ def basis_fd(btype=None, rangeval=None, nbasis=None, params=None, dropind=None,
         }
         return basisobj
 
-    
\ No newline at end of file
+