diff --git a/i2c-internal.h b/i2c-internal.h
index 95d245c..d2bc758 100644
--- a/i2c-internal.h
+++ b/i2c-internal.h
@@ -35,8 +35,10 @@
 
 	uint8_t own_addr;
 
-	uint8_t tx_storage[MCTP_PKTBUF_SIZE(I2C_BTU)] PKTBUF_STORAGE_ALIGN;
-	uint8_t rx_storage[MCTP_PKTBUF_SIZE(I2C_BTU)] PKTBUF_STORAGE_ALIGN;
+	uint8_t tx_storage[sizeof(struct mctp_i2c_hdr) +
+			   MCTP_PKTBUF_SIZE(I2C_BTU)] PKTBUF_STORAGE_ALIGN;
+	uint8_t rx_storage[sizeof(struct mctp_i2c_hdr) +
+			   MCTP_PKTBUF_SIZE(I2C_BTU)] PKTBUF_STORAGE_ALIGN;
 
 	mctp_i2c_tx_fn tx_fn;
 	void *tx_ctx;