diff --git a/beets/test/_common.py b/beets/test/_common.py
index 13fb26bb2..b0311ec46 100644
--- a/beets/test/_common.py
+++ b/beets/test/_common.py
@@ -215,6 +215,7 @@ class DummyIO:
         return self.stdin.reads
 
     def install(self):
+        assert not self.installed, "DummyIO already installed"
         self.installed = True
 
         self.orig_stdin = sys.stdin