diff --git a/src/OpenSearch.Client/Mapping/Visitor/PropertyWalker.cs b/src/OpenSearch.Client/Mapping/Visitor/PropertyWalker.cs
index fcd72b7415..c95dcb9812 100644
--- a/src/OpenSearch.Client/Mapping/Visitor/PropertyWalker.cs
+++ b/src/OpenSearch.Client/Mapping/Visitor/PropertyWalker.cs
@@ -205,11 +205,27 @@ private static Type GetUnderlyingType(Type type)
 			if (type.IsArray)
 				return type.GetElementType();
 
-			if (type.IsGenericType && type.GetGenericArguments().Length == 1
-				&& (type.GetInterfaces().HasAny(t => t == typeof(IEnumerable)) || Nullable.GetUnderlyingType(type) != null))
-				return type.GetGenericArguments()[0];
+			if (ShouldUnwrapType(type))
+			{
+				var returnType = type.GetGenericArguments()[0];
+				if (ShouldUnwrapType(returnType)) // This is needed for types like IEnumerable<int?>.
+				{
+					return returnType.GetGenericArguments()[0];
+				}
+				return returnType;
+			}
 
 			return type;
+
+			bool ShouldUnwrapType(Type ty)
+			{
+				return	ty.IsGenericType &&
+						ty.GetGenericArguments().Length == 1
+						&& (
+							ty.GetInterfaces().HasAny(t => t == typeof(IEnumerable)) ||
+							Nullable.GetUnderlyingType(ty) != null
+						);
+			}
 		}
 	}
 }