Index: src/OFZooArchive.m
==================================================================
--- src/OFZooArchive.m
+++ src/OFZooArchive.m
@@ -30,10 +30,11 @@
 #import "OFString.h"
 
 #import "OFChecksumMismatchException.h"
 #import "OFInvalidArgumentException.h"
 #import "OFInvalidFormatException.h"
+#import "OFNotImplementedException.h"
 #import "OFNotOpenException.h"
 #import "OFTruncatedDataException.h"
 #import "OFUnsupportedVersionException.h"
 
 enum {
@@ -85,10 +86,14 @@
 	self = [super init];
 
 	@try {
 		if ([mode isEqual: @"r"])
 			_mode = modeRead;
+		else if ([mode isEqual: @"w"] || [mode isEqual: @"a"])
+			@throw [OFNotImplementedException
+			    exceptionWithSelector: _cmd
+					   object: nil];
 		else
 			@throw [OFInvalidArgumentException exception];
 
 		_stream = [stream retain];
 		_encoding = OFStringEncodingUTF8;