Index: src/OFTarArchive.h
==================================================================
--- src/OFTarArchive.h
+++ src/OFTarArchive.h
@@ -27,11 +27,11 @@
  *
  * @brief A class for accessing and manipulating tar archives.
  */
 @interface OFTarArchive: OFObject
 {
-	OFStream *_stream;
+	OF_KINDOF(OFStream *) _stream;
 	enum {
 		OF_TAR_ARCHIVE_MODE_READ,
 		OF_TAR_ARCHIVE_MODE_WRITE,
 		OF_TAR_ARCHIVE_MODE_APPEND
 	} _mode;
@@ -40,17 +40,17 @@
 
 /*!
  * @brief Creates a new OFTarArchive object with the specified stream.
  *
  * @param stream A stream from which the tar archive will be read.
- *		 For append mode, this needs to be a seekable stream.
+ *		 For append mode, this needs to be an OFSeekableStream.
  * @param mode The mode for the tar file. Valid modes are "r" for reading,
  *	       "w" for creating a new file and "a" for appending to an existing
  *	       archive.
  * @return A new, autoreleased OFTarArchive
  */
-+ (instancetype)archiveWithStream: (OFStream *)stream
++ (instancetype)archiveWithStream: (OF_KINDOF(OFStream *))stream
 			     mode: (OFString *)mode;
 
 #ifdef OF_HAVE_FILES
 /*!
  * @brief Creates a new OFTarArchive object with the specified file.
@@ -68,17 +68,17 @@
 /*!
  * @brief Initializes an already allocated OFTarArchive object with the
  *	  specified stream.
  *
  * @param stream A stream from which the tar archive will be read.
- *		 For append mode, this needs to be a seekable stream.
+ *		 For append mode, this needs to be an OFSeekableStream.
  * @param mode The mode for the tar file. Valid modes are "r" for reading,
  *	       "w" for creating a new file and "a" for appending to an existing
  *	       archive.
  * @return An initialized OFTarArchive
  */
-- initWithStream: (OFStream *)stream
+- initWithStream: (OF_KINDOF(OFStream *))stream
 	    mode: (OFString *)mode OF_DESIGNATED_INITIALIZER;
 
 #ifdef OF_HAVE_FILES
 /*!
  * @brief Initializes an already allocated OFTarArchive object with the

Index: src/OFTarArchive.m
==================================================================
--- src/OFTarArchive.m
+++ src/OFTarArchive.m
@@ -71,11 +71,11 @@
 	for (size_t i = UTF8StringLength; i < length; i++)
 		buffer[i] = '\0';
 }
 
 @implementation OFTarArchive: OFObject
-+ (instancetype)archiveWithStream: (OFStream *)stream
++ (instancetype)archiveWithStream: (OF_KINDOF(OFStream *))stream
 			     mode: (OFString *)mode
 {
 	return [[[self alloc] initWithStream: stream
 					mode: mode] autorelease];
 }
@@ -87,11 +87,11 @@
 	return [[[self alloc] initWithPath: path
 				      mode: mode] autorelease];
 }
 #endif
 
-- initWithStream: (OFStream *)stream
+- initWithStream: (OF_KINDOF(OFStream *))stream
 	    mode: (OFString *)mode
 {
 	self = [super init];
 
 	@try {
@@ -114,12 +114,12 @@
 			bool empty = true;
 
 			if (![_stream isKindOfClass: [OFSeekableStream class]])
 				@throw [OFInvalidArgumentException exception];
 
-			[(OFSeekableStream *)stream seekToOffset: -1024
-							  whence: SEEK_END];
+			[stream seekToOffset: -1024
+				      whence: SEEK_END];
 			[stream readIntoBuffer: buffer.c
 				   exactLength: 1024];
 
 			for (size_t i = 0; i < 1024 / sizeof(uint32_t); i++)
 				if (buffer.u32[i] != 0)
@@ -126,12 +126,12 @@
 					empty = false;
 
 			if (!empty)
 				@throw [OFInvalidFormatException exception];
 
-			[(OFSeekableStream *)stream seekToOffset: -1024
-							  whence: SEEK_END];
+			[stream seekToOffset: -1024
+				      whence: SEEK_END];
 		}
 	} @catch (id e) {
 		[self release];
 		@throw e;
 	}

Index: src/OFZIPArchive.h
==================================================================
--- src/OFZIPArchive.h
+++ src/OFZIPArchive.h
@@ -21,21 +21,20 @@
 OF_ASSUME_NONNULL_BEGIN
 
 @class OFArray OF_GENERIC(ObjectType);
 @class OFMutableArray OF_GENERIC(ObjectType);
 @class OFMutableDictionary OF_GENERIC(KeyType, ObjectType);
-@class OFSeekableStream;
 @class OFStream;
 
 /*!
  * @class OFZIPArchive OFZIPArchive.h ObjFW/OFZIPArchive.h
  *
  * @brief A class for accessing and manipulating ZIP files.
  */
 @interface OFZIPArchive: OFObject
 {
-	OFSeekableStream *_stream;
+	OF_KINDOF(OFStream *) _stream;
 	enum {
 		OF_ZIP_ARCHIVE_MODE_READ,
 		OF_ZIP_ARCHIVE_MODE_WRITE,
 		OF_ZIP_ARCHIVE_MODE_APPEND
 	} _mode;
@@ -54,20 +53,21 @@
  * The archive comment.
  */
 @property (readonly, nonatomic) OFString *archiveComment;
 
 /*!
- * @brief Creates a new OFZIPArchive object with the specified seekable stream.
+ * @brief Creates a new OFZIPArchive object with the specified stream.
  *
- * @param stream A seekable stream from which the ZIP archive will be read
+ * @param stream A stream from which the ZIP archive will be read.
+ *		 For read and append mode, this needs to be an OFSeekableStream.
  * @param mode The mode for the ZIP file. Valid modes are "r" for reading,
  *	       "w" for creating a new file and "a" for appending to an existing
  *	       archive.
  * @return A new, autoreleased OFZIPArchive
  */
-+ (instancetype)archiveWithSeekableStream: (OFSeekableStream *)stream
-				     mode: (OFString *)mode;
++ (instancetype)archiveWithStream: (OF_KINDOF(OFStream *))stream
+			     mode: (OFString *)mode;
 
 #ifdef OF_HAVE_FILES
 /*!
  * @brief Creates a new OFZIPArchive object with the specified file.
  *
@@ -83,20 +83,21 @@
 
 - init OF_UNAVAILABLE;
 
 /*!
  * @brief Initializes an already allocated OFZIPArchive object with the
- *	  specified seekable stream.
+ *	  specified stream.
  *
- * @param stream A seekable stream from which the ZIP archive will be read
+ * @param stream A stream from which the ZIP archive will be read.
+ *		 For read and append mode, this needs to be an OFSeekableStream.
  * @param mode The mode for the ZIP file. Valid modes are "r" for reading,
  *	       "w" for creating a new file and "a" for appending to an existing
  *	       archive.
  * @return An initialized OFZIPArchive
  */
-- initWithSeekableStream: (OFSeekableStream *)stream
-		    mode: (OFString *)mode OF_DESIGNATED_INITIALIZER;
+- initWithStream: (OF_KINDOF(OFStream *))stream
+	    mode: (OFString *)mode OF_DESIGNATED_INITIALIZER;
 
 #ifdef OF_HAVE_FILES
 /*!
  * @brief Initializes an already allocated OFZIPArchive object with the
  *	  specified file.

Index: src/OFZIPArchive.m
==================================================================
--- src/OFZIPArchive.m
+++ src/OFZIPArchive.m
@@ -22,10 +22,11 @@
 #import "OFZIPArchiveEntry.h"
 #import "OFZIPArchiveEntry+Private.h"
 #import "OFData.h"
 #import "OFArray.h"
 #import "OFDictionary.h"
+#import "OFStream.h"
 #import "OFSeekableStream.h"
 #ifdef OF_HAVE_FILES
 # import "OFFile.h"
 #endif
 #import "OFInflateStream.h"
@@ -134,15 +135,15 @@
 }
 
 @implementation OFZIPArchive
 @synthesize archiveComment = _archiveComment;
 
-+ (instancetype)archiveWithSeekableStream: (OFSeekableStream *)stream
-				     mode: (OFString *)mode
++ (instancetype)archiveWithStream: (OF_KINDOF(OFStream *))stream
+			     mode: (OFString *)mode
 {
-	return [[[self alloc] initWithSeekableStream: stream
-						mode: mode] autorelease];
+	return [[[self alloc] initWithStream: stream
+					mode: mode] autorelease];
 }
 
 #ifdef OF_HAVE_FILES
 + (instancetype)archiveWithPath: (OFString *)path
 			   mode: (OFString *)mode
@@ -155,19 +156,22 @@
 - init
 {
 	OF_INVALID_INIT_METHOD
 }
 
-- initWithSeekableStream: (OFSeekableStream *)stream
-		    mode: (OFString *)mode
+- initWithStream: (OF_KINDOF(OFStream *))stream
+	    mode: (OFString *)mode
 {
 	self = [super init];
 
 	@try {
 		_stream = [stream retain];
 
 		if ([mode isEqual: @"r"]) {
+			if (![stream isKindOfClass: [OFSeekableStream class]])
+				@throw [OFInvalidArgumentException exception];
+
 			_mode = OF_ZIP_ARCHIVE_MODE_READ;
 
 			[self of_readZIPInfo];
 			[self of_readEntries];
 		} else if ([mode isEqual: @"w"] || [mode isEqual: @"a"])
@@ -189,12 +193,12 @@
 	  mode: (OFString *)mode
 {
 	OFFile *file = [[OFFile alloc] initWithPath: path
 					       mode: mode];
 	@try {
-		self = [self initWithSeekableStream: file
-					       mode: mode];
+		self = [self initWithStream: file
+				       mode: mode];
 	} @finally {
 		[file release];
 	}
 
 	return self;

Index: utils/ofzip/ZIPArchive.m
==================================================================
--- utils/ofzip/ZIPArchive.m
+++ utils/ofzip/ZIPArchive.m
@@ -67,12 +67,12 @@
 	    mode: (OFString *)mode
 {
 	self = [super init];
 
 	@try {
-		_archive = [[OFZIPArchive alloc] initWithSeekableStream: stream
-								   mode: mode];
+		_archive = [[OFZIPArchive alloc] initWithStream: stream
+							   mode: mode];
 	} @catch (id e) {
 		[self release];
 		@throw e;
 	}