diff --git a/consul/callback.py b/consul/callback.py
index d4637a2..f7bb1f8 100644
--- a/consul/callback.py
+++ b/consul/callback.py
@@ -72,21 +72,23 @@ def cb(response):
             if response.code == 404:
                 data = None
             else:
-                data = json.loads(response.body)
-                if decode:
-                    for item in data:
-                        if item.get(decode) is not None:
-                            item[decode] = base64.b64decode(item[decode])
-                if is_id:
-                    data = data["ID"]
-                if one:
-                    if data == []:
-                        data = None
-                    if data is not None:
-                        data = data[0]
-                if postprocess:
-                    data = postprocess(data)
+                try:
+                    data = json.loads(response.body)
+                    if decode:
+                        for item in data:
+                            if item.get(decode) is not None:
+                                item[decode] = base64.b64decode(item[decode])
+                    if is_id:
+                        data = data["ID"]
+                    if one and isinstance(data, list):
+                        data = data[0] if data else None
+                    if postprocess:
+                        data = postprocess(data)
+                except (json.JSONDecodeError, TypeError, KeyError) as e:
+                    raise ConsulException(f"Failed to decode JSON: {response.body} {e}") from e
             if index:
+                if "X-Consul-Index" not in response.headers:
+                    raise ConsulException(f"Missing index header: {response.headers}")
                 return response.headers["X-Consul-Index"], data
             return data