diff --git a/netjsonconfig/exceptions.py b/netjsonconfig/exceptions.py
index 8a38a3439..b4393af00 100644
--- a/netjsonconfig/exceptions.py
+++ b/netjsonconfig/exceptions.py
@@ -7,6 +7,10 @@ def _list_errors(e):
     :param e: ``jsonschema.exceptions.ValidationError`` instance
     """
     error_list = []
+    if not getattr(e, "context", None):
+        return error_list
+    if not getattr(e, "validator_value", None):
+        return error_list
     for value, error in zip(e.validator_value, e.context):
         error_list.append((value, error.message))
         if error.context:
@@ -25,6 +29,8 @@ def __str__(self):
             self.details,
         )
         errors = _list_errors(self.details)
+        if not errors:
+            return message
         separator = "\nAgainst schema %s\n%s\n"
         details = reduce(lambda x, y: x + separator % y, errors, "")
         return message + details
diff --git a/netjsonconfig/utils.py b/netjsonconfig/utils.py
index de9a08bf9..617e2d681 100644
--- a/netjsonconfig/utils.py
+++ b/netjsonconfig/utils.py
@@ -2,6 +2,10 @@
 from collections import OrderedDict
 from copy import deepcopy
 
+from jsonschema import ValidationError as JsonSchemaError
+
+from .exceptions import ValidationError
+
 
 def merge_config(template, config, list_identifiers=None):
     """
@@ -19,14 +23,27 @@ def merge_config(template, config, list_identifiers=None):
     :param config: config ``dict``
     :param list_identifiers: ``list`` or ``None``
     :returns: merged ``dict``
+    :raises ValidationError: if incompatible types are found
     """
     result = deepcopy(template)
     for key, value in config.items():
-        if isinstance(value, dict):
-            node = result.get(key, OrderedDict())
-            result[key] = merge_config(node, value)
-        elif isinstance(value, list) and isinstance(result.get(key), list):
-            result[key] = merge_list(result[key], value, list_identifiers)
+        existing = result.get(key)
+        if isinstance(value, dict) and isinstance(existing, dict):
+            result[key] = merge_config(existing, value, list_identifiers)
+        elif isinstance(value, list) and isinstance(existing, list):
+            result[key] = merge_list(existing, value, list_identifiers)
+        elif (
+            existing is not None
+            and isinstance(existing, (dict, list))
+            and isinstance(value, (dict, list))
+            and type(value) is not type(existing)
+        ):
+            raise ValidationError(
+                JsonSchemaError(
+                    f"Incompatible type for '{key}': expected {type(existing).__name__}, "
+                    f"got {type(value).__name__}."
+                )
+            )
         else:
             result[key] = value
     return result
diff --git a/tests/openwrt/test_default.py b/tests/openwrt/test_default.py
index 61ed40210..a22802b5b 100644
--- a/tests/openwrt/test_default.py
+++ b/tests/openwrt/test_default.py
@@ -3,6 +3,7 @@
 from openwisp_utils.tests import capture_stdout
 
 from netjsonconfig import OpenWrt
+from netjsonconfig.exceptions import ValidationError
 from netjsonconfig.utils import _TabsMixin
 
 
@@ -245,3 +246,28 @@ def test_render_invalid_uci_name(self):
     option lan '0.0.0.0/24 domain=1'
 """)
         self.assertEqual(o.render(), expected)
+
+    def test_merge_invalid_format(self):
+        invalid = {
+            "dhcp": {
+                "lan": {
+                    "interface": "lan",
+                    "start": 100,
+                    "limit": 150,
+                    "leasetime": "12h",
+                }
+            }
+        }
+        valid = {
+            "dhcp": [
+                {
+                    "dhcpv6": "disabled",
+                    "ignore": True,
+                    "ra": "disabled",
+                    "config_value": "lan",
+                    "config_name": "dhcp",
+                }
+            ]
+        }
+        with self.assertRaises(ValidationError):
+            OpenWrt({}, templates=[valid, invalid])
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 0d83b5d13..052734b7a 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -1,5 +1,6 @@
 import unittest
 
+from netjsonconfig.exceptions import ValidationError
 from netjsonconfig.utils import evaluate_vars, get_copy, merge_config, merge_list
 
 
@@ -55,6 +56,20 @@ def test_merge_list_of_dicts_unchanged(self):
             result, {"list": [{"a": "original"}, {"b": "changed"}, {"c": "original"}]}
         )
 
+    def test_merge_config_list_scalar_overrides(self):
+        template = {"server": ["1.1.1.1"]}
+        config = {"server": "8.8.8.8"}
+        result = merge_config(template, config)
+        self.assertEqual(result, {"server": "8.8.8.8"})
+
+    def test_merge_config_incompatible_dict_list(self):
+        template = {"a": {"b": "c"}}
+        config = {"a": ["x"]}
+        with self.assertRaises(ValidationError) as context:
+            merge_config(template, config)
+        self.assertIn("Incompatible type", str(context.exception))
+        self.assertIn("ValidationError", str(context.exception))
+
     def test_evaluate_vars(self):
         self.assertEqual(evaluate_vars("{{ tz }}", {"tz": "UTC"}), "UTC")
         self.assertEqual(evaluate_vars("tz: {{ tz }}", {"tz": "UTC"}), "tz: UTC")