diff --git a/abl/reasoning/reasoner.py b/abl/reasoning/reasoner.py
index 724a8e4..7bd4944 100644
--- a/abl/reasoning/reasoner.py
+++ b/abl/reasoning/reasoner.py
@@ -4,8 +4,7 @@ import numpy as np
 from zoopt import Dimension, Objective, Opt, Parameter, Solution
 
 from ..structures import ListData
-from ..utils.utils import (calculate_revision_num, confidence_dist,
-                           hamming_dist, reform_idx)
+from ..utils.utils import calculate_revision_num, confidence_dist, hamming_dist, reform_idx
 from .base_kb import BaseKB
 
 
@@ -13,7 +12,7 @@ class ReasonerBase:
     def __init__(
         self,
         kb: BaseKB,
-        dist_func: str = "hamming",
+        dist_func: str = "confidence",
         mapping: Mapping = None,
         use_zoopt: bool = False,
     ):
@@ -25,7 +24,7 @@ class ReasonerBase:
         kb : BaseKB
             The knowledge base to be used for reasoning.
         dist_func : str, optional
-            The distance function to be used. Can be "hamming" or "confidence". Default is "hamming".
+            The distance function to be used. Can be "hamming" or "confidence". Default is "confidence".
         mapping : dict, optional
             A mapping of indices to labels. If None, a default mapping is generated.
         use_zoopt : bool, optional
@@ -37,8 +36,8 @@ class ReasonerBase:
             If the specified distance function is neither "hamming" nor "confidence".
         """
 
-        if not (dist_func == "hamming" or dist_func == "confidence"):
-            raise NotImplementedError  # Only hamming or confidence distance is available.
+        if dist_func not in ["hamming", "confidence"]:
+            raise NotImplementedError(f"The distance function '{dist_func}' is not implemented.")
 
         self.kb = kb
         self.dist_func = dist_func
@@ -46,7 +45,18 @@ class ReasonerBase:
         if mapping is None:
             self.mapping = {index: label for index, label in enumerate(self.kb.pseudo_label_list)}
         else:
+            if not isinstance(mapping, dict):
+                raise ValueError("mapping must be of type dict")
+
+            for key, value in mapping.items():
+                if not isinstance(key, int):
+                    raise ValueError("All keys in the mapping must be integers")
+
+                if value not in self.kb.pseudo_label_list:
+                    raise ValueError("All values in the mapping must be in the pseudo_label_list")
+
             self.mapping = mapping
+
         self.remapping = dict(zip(self.mapping.values(), self.mapping.keys()))
 
     def _get_cost_list(self, data_sample: ListData, candidates: List[List[Any]]):