diff --git a/xcode/OLMKit/OLMAccount.m b/xcode/OLMKit/OLMAccount.m
index b194a74..2f30fb6 100644
--- a/xcode/OLMKit/OLMAccount.m
+++ b/xcode/OLMKit/OLMAccount.m
@@ -74,7 +74,7 @@
     return self;
 }
 
-- (size_t) maxOneTimeKeys {
+- (NSUInteger) maxOneTimeKeys {
     return olm_account_max_number_of_one_time_keys(_account);
 }
 
@@ -156,7 +156,7 @@
 - (BOOL) removeOneTimeKeysForSession:(OLMSession *)session {
     NSParameterAssert(session != nil);
     if (!session) {
-        return nil;
+        return NO;
     }
     size_t result = olm_remove_one_time_keys(self.account, session.session);
     if (result == olm_error()) {
diff --git a/xcode/OLMKit/OLMSession.m b/xcode/OLMKit/OLMSession.m
index e50da75..3801e08 100644
--- a/xcode/OLMKit/OLMSession.m
+++ b/xcode/OLMKit/OLMSession.m
@@ -167,9 +167,9 @@
 }
 
 - (BOOL)matchesInboundSession:(NSString *)oneTimeKeyMessage {
-    NSData *otk = [oneTimeKeyMessage dataUsingEncoding:NSUTF8StringEncoding];
+    NSMutableData *otk = [NSMutableData dataWithData:[oneTimeKeyMessage dataUsingEncoding:NSUTF8StringEncoding]];
 
-    size_t result = olm_matches_inbound_session(_session, otk.bytes, otk.length);
+    size_t result = olm_matches_inbound_session(_session, otk.mutableBytes, otk.length);
     if (result == 1) {
         return YES;
     }
@@ -184,11 +184,11 @@
 
 - (BOOL)matchesInboundSessionFrom:(NSString *)theirIdentityKey oneTimeKeyMessage:(NSString *)oneTimeKeyMessage {
     NSData *idKey = [theirIdentityKey dataUsingEncoding:NSUTF8StringEncoding];
-    NSData *otk = [oneTimeKeyMessage dataUsingEncoding:NSUTF8StringEncoding];
+    NSMutableData *otk = [NSMutableData dataWithData:[oneTimeKeyMessage dataUsingEncoding:NSUTF8StringEncoding]];
 
     size_t result = olm_matches_inbound_session_from(_session,
                                                      idKey.bytes, idKey.length,
-                                                     otk.bytes, otk.length);
+                                                     otk.mutableBytes, otk.length);
     if (result == 1) {
         return YES;
     }