diff --git a/chrome/content/zotero/preferences/preferences.js b/chrome/content/zotero/preferences/preferences.js
index 2ff21cc82f..e8e309af30 100644
--- a/chrome/content/zotero/preferences/preferences.js
+++ b/chrome/content/zotero/preferences/preferences.js
@@ -487,6 +487,7 @@ ${str}
 			}
 			if (this._mutationObservers.has(elem)) {
 				this._mutationObservers.get(elem).disconnect();
+				this._mutationObservers.delete(elem);
 			}
 		};