diff --git a/src/shearwater_common.c b/src/shearwater_common.c
index fef8460..f3cdc48 100644
--- a/src/shearwater_common.c
+++ b/src/shearwater_common.c
@@ -256,7 +256,7 @@ shearwater_common_slip_read (shearwater_common_device_t *device, unsigned char d
 }
 
 
-static dc_status_t
+dc_status_t
 shearwater_common_transfer (shearwater_common_device_t *device, const unsigned char input[], unsigned int isize, unsigned char output[], unsigned int osize, unsigned int *actual)
 {
 	dc_device_t *abstract = (dc_device_t *) device;
@@ -283,6 +283,13 @@ shearwater_common_transfer (shearwater_common_device_t *device, const unsigned c
 			return DC_STATUS_TIMEOUT;
 	}
 
+	// Return early if no response packet is requested.
+	if (osize == 0) {
+		if (actual)
+			*actual = 0;
+		return DC_STATUS_SUCCESS;
+	}
+
 	// Receive the response packet.
 	n = shearwater_common_slip_read (device, packet, sizeof (packet));
 	if (n <= 0 || n > sizeof (packet)) {
@@ -309,7 +316,8 @@ shearwater_common_transfer (shearwater_common_device_t *device, const unsigned c
 	}
 
 	memcpy (output, packet + 4, length - 1);
-	*actual = length - 1;
+	if (actual)
+		*actual = length - 1;
 
 	return DC_STATUS_SUCCESS;
 }
diff --git a/src/shearwater_common.h b/src/shearwater_common.h
index 78aa6c7..dbb717e 100644
--- a/src/shearwater_common.h
+++ b/src/shearwater_common.h
@@ -40,6 +40,9 @@ shearwater_common_open (shearwater_common_device_t *device, dc_context_t *contex
 dc_status_t
 shearwater_common_close (shearwater_common_device_t *device);
 
+dc_status_t
+shearwater_common_transfer (shearwater_common_device_t *device, const unsigned char input[], unsigned int isize, unsigned char output[], unsigned int osize, unsigned int *actual);
+
 dc_status_t
 shearwater_common_download (shearwater_common_device_t *device, dc_buffer_t *buffer, unsigned int address, unsigned int size, unsigned int compression);
 
diff --git a/src/shearwater_petrel.c b/src/shearwater_petrel.c
index 5bc761d..ac84848 100644
--- a/src/shearwater_petrel.c
+++ b/src/shearwater_petrel.c
@@ -101,6 +101,10 @@ shearwater_petrel_device_close (dc_device_t *abstract)
 	dc_status_t rc = DC_STATUS_SUCCESS;
 	shearwater_common_device_t *device = (shearwater_common_device_t *) abstract;
 
+	// Shutdown the device.
+	unsigned char request[] = {0x2E, 0x90, 0x20, 0x00};
+	shearwater_common_transfer (device, request, sizeof (request), NULL, 0, NULL);
+
 	// Close the device.
 	rc = shearwater_common_close (device);