diff --git a/splink/comparison_template_library.py b/splink/comparison_template_library.py
index 01ba73dbb5..ffb23fa00b 100644
--- a/splink/comparison_template_library.py
+++ b/splink/comparison_template_library.py
@@ -182,7 +182,7 @@ def __init__(
             )
 
             if m_probability_1st_january:
-                comparison_level["m_probability"] = m_probability_1st_january
+                comparison_level.m_probability = m_probability_1st_january
             if term_frequency_adjustments:
                 comparison_level["tf_adjustment_column"] = col_name
             comparison_levels.append(comparison_level)