diff --git a/mastodon/info.py b/mastodon/info.py
index ecd247d..2b97559 100644
--- a/mastodon/info.py
+++ b/mastodon/info.py
@@ -54,6 +54,12 @@ class InstanceInfo:
                     config["text_format"] = "text/x.misskeymarkdown"
                 elif "text/markdown" in text_mimes:
                     config["text_format"] = "text/markdown"
+                elif "text/plain" in text_mimes:
+                    config["text_format"] = "text/plain"
+                else:
+                    raise ValueError(
+                        f"Unsupported instance 'text_format' type '{text_mimes}'"
+                    )
 
         if "media_attachments" in data:
             media_config: dict[str, Any] = data["media_attachments"]
@@ -80,6 +86,12 @@ class InstanceInfo:
                         config["text_format"] = "text/x.misskeymarkdown"
                     elif "text/markdown" in post_formats:
                         config["text_format"] = "text/markdown"
+                    elif "text/plain" in post_formats:
+                        config["text_format"] = "text/plain"
+                    else:
+                        raise ValueError(
+                            f"Unsupported instance 'text_format' type '{post_formats}'"
+                        )
 
         return InstanceInfo(**config)