diff --git a/pycbc/fft/core.py b/pycbc/fft/core.py
index 92e3cdcb229..3c15d5653a3 100644
--- a/pycbc/fft/core.py
+++ b/pycbc/fft/core.py
@@ -98,7 +98,7 @@ def _check_fwd_args(invec, itype, outvec, otype, nbatch, size):
     olen = len(outvec)
     if nbatch < 1:
         raise ValueError("nbatch must be >= 1")
-    if (nbatch > 1) and size is not None:
+    if (nbatch > 1) and size is None:
         raise ValueError("When nbatch > 1, size cannot be 'None'")
     if size is None:
         size = ilen