diff --git a/reciprocalspaceship/dataset.py b/reciprocalspaceship/dataset.py
index 4be2b565..aa187a49 100644
--- a/reciprocalspaceship/dataset.py
+++ b/reciprocalspaceship/dataset.py
@@ -1214,7 +1214,7 @@ def is_isomorphous(self, other, cell_threshold=0.5):
         for param in params:
             param1 = self.cell.__getattribute__(param)
             param2 = other.cell.__getattribute__(param)
-            diff = 200. * np.abs(param1 - param2) / (param1 + param2)
+            diff = 200.0 * np.abs(param1 - param2) / (param1 + param2)
             if diff > cell_threshold:
                 return False