diff --git a/lmms_eval/tasks/mmbench/cc_utils.py b/lmms_eval/tasks/mmbench/cc_utils.py
index d64f587b..eb64d0f2 100644
--- a/lmms_eval/tasks/mmbench/cc_utils.py
+++ b/lmms_eval/tasks/mmbench/cc_utils.py
@@ -62,7 +62,7 @@ def mmbench_cn_cc_process_results(doc, results):
     }
     option_candidate = ["A", "B", "C", "D", "E"]
     for c in option_candidate:
-        data[c] = doc.get(c, None)
+        data[c] = doc.get(c, "nan")
     return data
 
 
diff --git a/lmms_eval/tasks/mmbench/cn_utils.py b/lmms_eval/tasks/mmbench/cn_utils.py
index 99af01c6..72b28e8d 100644
--- a/lmms_eval/tasks/mmbench/cn_utils.py
+++ b/lmms_eval/tasks/mmbench/cn_utils.py
@@ -69,7 +69,7 @@ def mmbench_process_results(doc, results):
     }
     option_candidate = ["A", "B", "C", "D", "E"]
     for c in option_candidate:
-        data[c] = doc.get(c, None)
+        data[c] = doc.get(c, "nan")
     return data
 
 
diff --git a/lmms_eval/tasks/mmbench/en_utils.py b/lmms_eval/tasks/mmbench/en_utils.py
index dc4541c0..c0700405 100644
--- a/lmms_eval/tasks/mmbench/en_utils.py
+++ b/lmms_eval/tasks/mmbench/en_utils.py
@@ -69,7 +69,7 @@ def mmbench_process_results(doc, results):
     }
     option_candidate = ["A", "B", "C", "D", "E"]
     for c in option_candidate:
-        data[c] = doc.get(c, None)
+        data[c] = doc.get(c, "nan")
     return data