diff --git a/src/network/network_server.cpp b/src/network/network_server.cpp
index 97a1e57d6c..c720ae18f4 100644
--- a/src/network/network_server.cpp
+++ b/src/network/network_server.cpp
@@ -141,7 +141,7 @@ struct PacketWriter : SaveFilter {
 		return false;
 	}
 
-	void Write(uint8_t *buf, size_t size) override
+	void Write(const uint8_t *buf, size_t size) override
 	{
 		std::lock_guard<std::mutex> lock(this->mutex);
 
diff --git a/src/saveload/saveload.cpp b/src/saveload/saveload.cpp
index d5095d3b08..3f82736ecd 100644
--- a/src/saveload/saveload.cpp
+++ b/src/saveload/saveload.cpp
@@ -2429,7 +2429,7 @@ struct FileWriter : SaveFilter {
 		this->Finish();
 	}
 
-	void Write(uint8_t *buf, size_t size) override
+	void Write(const uint8_t *buf, size_t size) override
 	{
 		/* We're in the process of shutting down, i.e. in "failure" mode. */
 		if (!this->file.has_value()) return;
@@ -2513,7 +2513,7 @@ struct LZOSaveFilter : SaveFilter {
 		if (lzo_init() != LZO_E_OK) SlError(STR_GAME_SAVELOAD_ERROR_BROKEN_INTERNAL_ERROR, "cannot initialize compressor");
 	}
 
-	void Write(uint8_t *buf, size_t size) override
+	void Write(const uint8_t *buf, size_t size) override
 	{
 		const lzo_bytep in = buf;
 		/* Buffer size is from the LZO docs plus the chunk header size. */
@@ -2568,7 +2568,7 @@ struct NoCompSaveFilter : SaveFilter {
 	{
 	}
 
-	void Write(uint8_t *buf, size_t size) override
+	void Write(const uint8_t *buf, size_t size) override
 	{
 		this->chain->Write(buf, size);
 	}
@@ -2650,10 +2650,10 @@ struct ZlibSaveFilter : SaveFilter {
 	 * @param len  Amount of bytes to write.
 	 * @param mode Mode for deflate.
 	 */
-	void WriteLoop(uint8_t *p, size_t len, int mode)
+	void WriteLoop(const uint8_t *p, size_t len, int mode)
 	{
 		uint n;
-		this->z.next_in = p;
+		this->z.next_in = const_cast<uint8_t *>(p); // zlib does not modify the data, but is non-const for legacy reasons
 		this->z.avail_in = static_cast<uInt>(len);
 		do {
 			this->z.next_out = this->fwrite_buf;
@@ -2678,7 +2678,7 @@ struct ZlibSaveFilter : SaveFilter {
 		} while (this->z.avail_in || !this->z.avail_out);
 	}
 
-	void Write(uint8_t *buf, size_t size) override
+	void Write(const uint8_t *buf, size_t size) override
 	{
 		this->WriteLoop(buf, size, 0);
 	}
@@ -2776,7 +2776,7 @@ struct LZMASaveFilter : SaveFilter {
 	 * @param len    Amount of bytes to write.
 	 * @param action Action for lzma_code.
 	 */
-	void WriteLoop(uint8_t *p, size_t len, lzma_action action)
+	void WriteLoop(const uint8_t *p, size_t len, lzma_action action)
 	{
 		size_t n;
 		this->lzma.next_in = p;
@@ -2796,7 +2796,7 @@ struct LZMASaveFilter : SaveFilter {
 		} while (this->lzma.avail_in || !this->lzma.avail_out);
 	}
 
-	void Write(uint8_t *buf, size_t size) override
+	void Write(const uint8_t *buf, size_t size) override
 	{
 		this->WriteLoop(buf, size, LZMA_RUN);
 	}
diff --git a/src/saveload/saveload_filter.h b/src/saveload/saveload_filter.h
index e2d030976e..4a0f0f7d3f 100644
--- a/src/saveload/saveload_filter.h
+++ b/src/saveload/saveload_filter.h
@@ -75,7 +75,7 @@ struct SaveFilter {
 	 * @param buf The bytes to write.
 	 * @param len The number of bytes to write.
 	 */
-	virtual void Write(uint8_t *buf, size_t len) = 0;
+	virtual void Write(const uint8_t *buf, size_t len) = 0;
 
 	/**
 	 * Prepare everything to finish writing the savegame.