diff --git a/mastodon/Mastodon.py b/mastodon/Mastodon.py
index fc585ba..37e35b7 100644
--- a/mastodon/Mastodon.py
+++ b/mastodon/Mastodon.py
@@ -2214,9 +2214,12 @@ class Mastodon:
             if not response_object.ok:
                 try:
                     response = response_object.json(object_hook=self.__json_hooks)
-                    if not isinstance(response, dict) or 'error' not in response:
+                    if isinstance(response, dict) and 'error' in response:
+                        error_msg = response['error']
+                    elif isinstance(response, str):
+                        error_msg = response
+                    else:
                         error_msg = None
-                    error_msg = response['error']
                 except ValueError:
                     error_msg = None
 
diff --git a/tests/test_errors.py b/tests/test_errors.py
new file mode 100644
index 0000000..7329507
--- /dev/null
+++ b/tests/test_errors.py
@@ -0,0 +1,20 @@
+import pytest
+from mastodon.Mastodon import MastodonAPIError
+
+try:
+    from mock import MagicMock
+except ImportError:
+    from unittest.mock import MagicMock
+
+def test_nonstandard_errors(api):
+    response = MagicMock()
+    response.json = MagicMock(return_value=
+            "I am a non-standard instance and this error is a plain string.")
+    response.ok = False
+    session = MagicMock()
+    session.request = MagicMock(return_value=response)
+
+    api.session = session
+    with pytest.raises(MastodonAPIError):
+        api.instance()
+