diff --git a/bilby/core/prior/dict.py b/bilby/core/prior/dict.py
index 3ac54622e..6ee29d14e 100644
--- a/bilby/core/prior/dict.py
+++ b/bilby/core/prior/dict.py
@@ -461,7 +461,7 @@ def check_efficiency(n_tested, n_valid):
                 sample = self.sample_subset(keys=keys, size=size)
                 is_valid = self.evaluate_constraints(sample)
                 n_tested_samples += 1
-                n_valid_samples += int(is_valid)
+                n_valid_samples += int(np.squeeze(is_valid))
                 check_efficiency(n_tested_samples, n_valid_samples)
                 if is_valid:
                     return sample