diff --git a/src/XMPPRoster.h b/src/XMPPRoster.h
index c05e52f..310234e 100644
--- a/src/XMPPRoster.h
+++ b/src/XMPPRoster.h
@@ -73,6 +73,7 @@
 	OFMutableDictionary *rosterItems;
 	XMPPMulticastDelegate *delegates;
 	id <XMPPStorage> dataStorage;
+	BOOL rosterRequested;
 /// \endcond
 }
 
diff --git a/src/XMPPRoster.m b/src/XMPPRoster.m
index 5b52719..4e76176 100644
--- a/src/XMPPRoster.m
+++ b/src/XMPPRoster.m
@@ -29,6 +29,8 @@
 
 #include <assert.h>
 
+#import <ObjFW/OFInvalidArgumentException.h>
+
 #import "XMPPRoster.h"
 #import "XMPPRosterItem.h"
 #import "XMPPConnection.h"
@@ -74,6 +76,8 @@
 {
 	XMPPIQ *iq;
 
+	rosterRequested = YES;
+
 	iq = [XMPPIQ IQWithType: @"get"
 			     ID: [connection generateStanzaID]];
 	[iq addChild: [OFXMLElement elementWithName: @"query"
@@ -188,7 +192,9 @@
 
 - (void)setDataStorage: (id <XMPPStorage>)dataStorage_
 {
-	/* TODO: Prevent changing it after it has been used */
+	if (rosterRequested)
+		@throw [OFInvalidArgumentException exceptionWithClass: isa];
+
 	dataStorage = dataStorage_;
 }