diff --git a/components/tidb_query_expr/src/impl_cast.rs b/components/tidb_query_expr/src/impl_cast.rs
index 53750d02d2d..6ce43695213 100644
--- a/components/tidb_query_expr/src/impl_cast.rs
+++ b/components/tidb_query_expr/src/impl_cast.rs
@@ -1385,8 +1385,9 @@ fn cast_string_as_json(
                 let mut vec;
                 if typ.tp() == FieldTypeTp::String {
                     vec = (*val).to_owned();
-                    // the `flen` of string is always greater than zero
-                    vec.resize(typ.flen().try_into().unwrap(), 0);
+                    if typ.flen() > 0 {
+                        vec.resize(typ.flen().try_into().unwrap(), 0);
+                    }
                     buf = &vec;
                 }
 
@@ -7021,6 +7022,17 @@ mod tests {
                 Json::from_opaque(FieldTypeTp::String, &[97]).unwrap(),
                 true,
             ),
+            (
+                FieldTypeBuilder::new()
+                    .tp(FieldTypeTp::VarChar)
+                    .flen(UNSPECIFIED_LENGTH)
+                    .charset(CHARSET_BIN)
+                    .collation(Collation::Binary)
+                    .build(),
+                "a".to_string(),
+                Json::from_opaque(FieldTypeTp::String, &[97]).unwrap(),
+                true,
+            ),
         ];
         for (arg_type, input, expect, parse_to_json) in cs {
             let arg_value = ScalarValue::Bytes(Some(input.clone().into_bytes()));