diff --git a/openfisca_core/commons/formulas.py b/openfisca_core/commons/formulas.py
index ccdeaa4732..21a269d28f 100644
--- a/openfisca_core/commons/formulas.py
+++ b/openfisca_core/commons/formulas.py
@@ -60,7 +60,7 @@ def apply_thresholds(
 
     assert len(condlist) == len(choices), \
         "apply_thresholds must be called with the same number of thresholds " \
-        "than choices, or one more choice"
+        "as choices, or with one additional choice."
 
     return numpy.select(condlist, choices)