diff --git a/pylogit/mixed_logit.py b/pylogit/mixed_logit.py
index <HASH>..<HASH> 100755
--- a/pylogit/mixed_logit.py
+++ b/pylogit/mixed_logit.py
@@ -419,8 +419,9 @@ class MixedLogit(base_mcm.MNDC_Model):
 
         if "intercept_ref_pos" in kwargs:
             if kwargs["intercept_ref_pos"] is not None:
-                msg = "All Mixed Logit intercepts should be in the index."
-                raise ValueError(msg)
+                msg = "All Mixed Logit intercepts should be in the index. "
+                msg_2 = "intercept_ref_pos should be None."
+                raise ValueError(msg + msg_2)
 
         # Carry out the common instantiation process for all choice models
         model_name = model_type_to_display_name["Mixed Logit"]
Changed the ValueError message for when an individual includes the intercept_ref_pos in the fit_mle method so that it is more specific in pointing out the offense.