Index: src/XMPPRoster.h
==================================================================
--- src/XMPPRoster.h
+++ src/XMPPRoster.h
@@ -71,10 +71,11 @@
 /// \cond internal
 	XMPPConnection *connection;
 	OFMutableDictionary *rosterItems;
 	XMPPMulticastDelegate *delegates;
 	id <XMPPStorage> dataStorage;
+	BOOL rosterRequested;
 /// \endcond
 }
 
 #ifdef OF_HAVE_PROPERTIES
 /**

Index: src/XMPPRoster.m
==================================================================
--- src/XMPPRoster.m
+++ src/XMPPRoster.m
@@ -27,10 +27,12 @@
 
 #define XMPP_ROSTER_M
 
 #include <assert.h>
 
+#import <ObjFW/OFInvalidArgumentException.h>
+
 #import "XMPPRoster.h"
 #import "XMPPRosterItem.h"
 #import "XMPPConnection.h"
 #import "XMPPIQ.h"
 #import "XMPPJID.h"
@@ -71,10 +73,12 @@
 }
 
 - (void)requestRoster
 {
 	XMPPIQ *iq;
+
+	rosterRequested = YES;
 
 	iq = [XMPPIQ IQWithType: @"get"
 			     ID: [connection generateStanzaID]];
 	[iq addChild: [OFXMLElement elementWithName: @"query"
 					  namespace: XMPP_NS_ROSTER]];
@@ -186,11 +190,13 @@
 	[delegates removeDelegate: delegate];
 }
 
 - (void)setDataStorage: (id <XMPPStorage>)dataStorage_
 {
-	/* TODO: Prevent changing it after it has been used */
+	if (rosterRequested)
+		@throw [OFInvalidArgumentException exceptionWithClass: isa];
+
 	dataStorage = dataStorage_;
 }
 
 - (id <XMPPStorage>)dataStorage
 {