diff --git a/isochrones/interp.py b/isochrones/interp.py
index 8cd6210..2cb5f59 100644
--- a/isochrones/interp.py
+++ b/isochrones/interp.py
@@ -629,7 +629,7 @@ def find_closest(self, val, lo, hi, v1, v2, col="initial_mass", debug=False):
             return find_closest3(val, lo, hi, v1, v2, self.grid, icol, *self.index_columns, debug=debug)
 
     def __call__(self, p, cols="all"):
-        if cols is "all":
+        if cols == "all":
             icols = np.arange(self.n_columns)
         else:
             icols = np.array([self.column_index[col] for col in cols])