diff --git a/swash/processing/post.py b/swash/processing/post.py
index 9032879..7fec275 100644
--- a/swash/processing/post.py
+++ b/swash/processing/post.py
@@ -31,9 +31,9 @@ x, t = data["x"], data["t"]
 
 # Cospectral calculations
 x0 = config.getint("post", "x0")
-arg_x0 = (x - x0).abs().argmin()
+arg_x0 = np.abs(x - x0).argmin()
 t0 = config.getfloat("post", "t0")
-arg_t0 = (t - t0).abs().argmin()
+arg_t0 = np.abs(t - t0).argmin()
 dt = config.getfloat("post", "dt")
 f = 1 / dt
 log.info(f"Computing reflection coefficient at x={x0}")