diff --git a/command.py b/command.py
index 55e21ca..6a49c2e 100644
--- a/command.py
+++ b/command.py
@@ -1,5 +1,14 @@
+import json
 import jsonschema
-from dataclasses import dataclass
+from dataclasses import dataclass, asdict
+
+
+class CommandArgumentError(ValueError):
+    pass
+
+
+class CommandExecutionError(RuntimeError):
+    pass
 
 
 class Command:
@@ -9,19 +18,25 @@ class Command:
         self.handler = handler
         self.additional_properties = kwargs
 
-        self._validator = jsonschema.validators.validator_for(
-            schema, default=jsonschema.validators.Draft7Validator
-        )(schema=schema)
-
-    def validate(self, o) -> bool:
-        return self._validator.is_valid(o)
-
-    def __call__(self, handler_instance, args):
-        if not self.validate(args):
-            raise ValueError(f"Invalid arguments for command '{self.name}'")
+    def __call__(self, handler_instance, payload):
         if self.handler is None:
-            raise RuntimeError(f"No handler bound for command '{self.name}'")
-        return self.handler(handler_instance, args)
+            raise NotImplementedError(f"No handler bound for command '{self.name}'")
+
+        try:
+            arguments = json.loads(payload)
+            jsonschema.validate(arguments, self.schema)
+        except json.decoder.JSONDecodeError as e:
+            raise CommandArgumentError(
+                f"Invalid JSON at line {e.lineno} column {e.colno}: {e.msg}"
+            )
+        except jsonschema.ValidationError as e:
+            raise CommandArgumentError(f"Schema error in {e.json_path}: {e.message}")
+
+        try:
+            return self.handler(handler_instance, arguments)
+        except Exception as e:
+            print("Internal command error: ", e)
+            raise CommandExecutionError(f"Command execution failed internally.")
 
 
 def command(schema, **kwargs):
@@ -36,3 +51,6 @@ class CommandResponse:
     success: bool
     message: str = None
     correlation: str = None
+
+    def __str__(self):
+        return json.dumps(asdict(self))
diff --git a/handler.py b/handler.py
index c57ea84..8362545 100644
--- a/handler.py
+++ b/handler.py
@@ -1,10 +1,14 @@
-import asyncio
 import aiomqtt
-import json
-from command import Command, CommandResponse
+import asyncio
 import inspect
-from paho.mqtt.properties import Properties
-from dataclasses import asdict
+import paho
+
+from command import (
+    Command,
+    CommandResponse,
+    CommandArgumentError,
+    CommandExecutionError,
+)
 
 
 class MQTTHandler:
@@ -47,9 +51,11 @@ class MQTTHandler:
                 )
 
     async def execute_command(
-        self, command_name: str, payload: str, properties: Properties = None
+        self,
+        command_name: str,
+        payload: str,
+        properties: paho.mqtt.properties.Properties = None,
     ):
-
         async def respond(success: bool, message: str = None):
             if (
                 properties is not None
@@ -63,23 +69,17 @@ class MQTTHandler:
                 )
                 await self.mqtt_client.publish(
                     properties.ResponseTopic,
-                    json.dumps(
-                        asdict(CommandResponse(success, str(message), correlation))
-                    ),
+                    str(CommandResponse(success, str(message), correlation)),
                     qos=1,
                     retain=False,
                 )
 
         try:
             command = self.get_available_commands()[command_name]
-            argument = json.loads(payload)
-            result = await command(self, argument)
+            result = await command(self, payload)
             await respond(True, result)
-
-        except json.decoder.JSONDecodeError as e:
-            await respond(False, f"Failed to parse payload as JSON: {e}")
-        except ValueError as e:
-            await respond(False, f"Command payload does not match expected schema: {e}")
+        except (CommandArgumentError, CommandExecutionError) as e:
+            await respond(False, f"{e}")
         except Exception as e:
             print(f"Failed to execute command {command_name} with unknown cause: ", e)
             await respond(False, "Unexpected error")
diff --git a/main.py b/main.py
index e729313..ea7bf12 100755
--- a/main.py
+++ b/main.py
@@ -5,6 +5,7 @@ import aioserial
 import asyncio
 import paho
 import uuid
+import socket
 
 from ubxhandler import UBXHandler
 
@@ -12,7 +13,7 @@ BAUD = 115200
 
 
 async def main():
-    handler_id = "example-gps"
+    handler_id = f"example-gps-{socket.gethostname()}"
     mqtt_host = "127.0.0.1"
     mqtt_port = 1883
 
@@ -30,7 +31,7 @@ async def main():
         timeout=0.05,  # 50 ms
     )
 
-    handler = UBXHandler(mqtt_client, "example-gps", serial_port)
+    handler = UBXHandler(mqtt_client, handler_id, serial_port)
     await handler.run()
 
 
diff --git a/ubxhandler.py b/ubxhandler.py
index 7a928a7..32b17cc 100644
--- a/ubxhandler.py
+++ b/ubxhandler.py
@@ -70,10 +70,10 @@ class UBXHandler(MQTTHandler):
                     if name.startswith("_"):
                         continue
 
-                    topic = f"{self.topic_base}/{message.identity}/{name}"
+                    topic = f"{self.property_topic}/{message.identity}/{name}"
                     await self.mqtt_client.publish(topic, value, qos=1, retain=True)
             else:
-                #print("Unexpected response:", message)
+                # print("Unexpected response:", message)
                 pass
 
     @command({"type": "number"}, description="An example command")