diff --git a/chrome/content/zotero/xpcom/http.js b/chrome/content/zotero/xpcom/http.js
index f0bf02260..3fc2521a8 100644
--- a/chrome/content/zotero/xpcom/http.js
+++ b/chrome/content/zotero/xpcom/http.js
@@ -14,6 +14,7 @@ Zotero.HTTP = new function() {
 		this.status = xmlhttp.status;
 		this.channel = xmlhttp.channel;
 		this.message = msg;
+		this.stack = new Error().stack;
 		
 		// Hide password from debug output
 		//
@@ -40,9 +41,6 @@ Zotero.HTTP = new function() {
 	this.UnexpectedStatusException.prototype.is5xx = function () {
 		return this.status >= 500 && this.status < 600;
 	}
-	this.UnexpectedStatusException.prototype.toString = function() {
-		return this.message;
-	};
 	
 	/**
 	 * Exception returned if the browser is offline when promise* is used
@@ -50,19 +48,15 @@ Zotero.HTTP = new function() {
 	 */
 	this.BrowserOfflineException = function() {
 		this.message = "XMLHttpRequest could not complete because the browser is offline";
+		this.stack = new Error().stack;
 	};
 	this.BrowserOfflineException.prototype = Object.create(Error.prototype);
-	this.BrowserOfflineException.prototype.toString = function() {
-		return this.message;
-	};
 
 	this.TimeoutException = function(ms) {
 		this.message = "XMLHttpRequest has timed out after " + ms + "ms";
+		this.stack = new Error().stack;
 	};
 	this.TimeoutException.prototype = Object.create(Error.prototype);
-	this.TimeoutException.prototype.toString = function() {
-		return this.message;
-	};
 
 	this.promise = function () {
 		Zotero.debug("Zotero.HTTP.promise() is deprecated -- use Zotero.HTTP.request()", 2);
diff --git a/chrome/content/zotero/xpcom/utilities.js b/chrome/content/zotero/xpcom/utilities.js
index 2f00a2380..15a91dda9 100644
--- a/chrome/content/zotero/xpcom/utilities.js
+++ b/chrome/content/zotero/xpcom/utilities.js
@@ -1431,10 +1431,24 @@ Zotero.Utilities = {
 				header = (obj.name ? obj.name + ' ' : '') + 'Exception';
 			}
 			
-			return header + ': '
-				+ (obj.message ? ('' + obj.message).replace(/^/gm, level_padding).trim() : '')
-				+ '\n\n'
-				+ (obj.stack ? obj.stack.trim().replace(/^(?=.)/gm, level_padding) : '');
+			let msg = (obj.message ? ('' + obj.message).replace(/^/gm, level_padding).trim() : '');
+			if (obj.stack) {
+				let stack = obj.stack.trim().replace(/^(?=.)/gm, level_padding);
+				
+				msg += '\n\n';
+				
+				// At least with Zotero.HTTP.UnexpectedStatusException, the stack contains "Error:"
+				// and the message in addition to the trace. I'm not sure what's causing that
+				// (Bluebird?), but fix it here.
+				if (obj.stack.startsWith('Error:')) {
+					msg += obj.stack.replace('Error: ' + obj.message + '\n', '');
+				}
+				else {
+					msg += stack;
+				}
+			}
+			
+			return header + ': ' + msg;
 		}
 		
 		// Only dump single level for nsIDOMNode objects (including document)