diff --git a/pylxd/exceptions.py b/pylxd/exceptions.py
index 54c06822..8afb1656 100644
--- a/pylxd/exceptions.py
+++ b/pylxd/exceptions.py
@@ -17,7 +17,13 @@ def __init__(self, response):
     def __str__(self):
         if self.response.status_code == 200:  # Operation failure
             try:
-                return self.response.json()["metadata"]["err"]
+                json_response = self.response.json()
+                metadata = json_response.get("metadata")
+                if metadata and isinstance(metadata, dict) and "err" in metadata:
+                    return json_response["metadata"]["err"]
+                if "error" in json_response:
+                    return json_response["error"]
+                return str(json_response)
             except (ValueError, KeyError):
                 pass
 
diff --git a/pylxd/models/tests/test_operation.py b/pylxd/models/tests/test_operation.py
index 8c2d6b7c..fc794fe4 100644
--- a/pylxd/models/tests/test_operation.py
+++ b/pylxd/models/tests/test_operation.py
@@ -100,6 +100,29 @@ def error(request, context):
 
         self.assertRaises(exceptions.LXDAPIException, an_operation.wait)
 
+    def test_wait_with_error_async_without_metadata(self):
+        """If the operation errors, wait raises an exception."""
+
+        def error(request, context):
+            context.status_code = 200
+            return {"type": "async", "error": "Could not proceed", "metadata": None}
+
+        self.add_rule(
+            {
+                "json": error,
+                "method": "GET",
+                "url": r"^http://pylxd.test/1.0/operations/operation-abc/wait$",
+            }
+        )
+
+        name = "/1.0/operations/operation-abc"
+
+        an_operation = models.Operation.get(self.client, name)
+
+        with self.assertRaises(exceptions.LXDAPIException) as wait_cm:
+            an_operation.wait()
+        assert str(wait_cm.exception) == "Could not proceed"
+
     def test_unknown_attribute(self):
         self.add_rule(
             {