diff --git a/src/oceanic_atom2.c b/src/oceanic_atom2.c
index af5e6e8..584b253 100644
--- a/src/oceanic_atom2.c
+++ b/src/oceanic_atom2.c
@@ -217,7 +217,7 @@ oceanic_atom2_quit (oceanic_atom2_device_t *device)
 	}
 
 	// Verify the answer.
-	if (answer[0] != 0xA5) {
+	if (answer[0] != NAK) {
 		WARNING ("Unexpected answer byte(s).");
 		return DEVICE_STATUS_PROTOCOL;
 	}
diff --git a/src/oceanic_vtpro.c b/src/oceanic_vtpro.c
index 1e3e2dd..6402af3 100644
--- a/src/oceanic_vtpro.c
+++ b/src/oceanic_vtpro.c
@@ -347,7 +347,7 @@ oceanic_vtpro_device_keepalive (device_t *abstract)
 	}
 
 	// Verify the answer.
-	if (answer[0] != 0x5A || answer[1] != 0x51) {
+	if (answer[0] != ACK || answer[1] != 0x51) {
 		WARNING ("Unexpected answer byte(s).");
 		return DEVICE_STATUS_PROTOCOL;
 	}
@@ -388,7 +388,7 @@ oceanic_vtpro_device_calibrate (device_t *abstract, unsigned char data[], unsign
 	}
 
 	// Verify the answer.
-	if (answer[0] != 0x5A || answer[2] != 0x00) {
+	if (answer[0] != ACK || answer[2] != 0x00) {
 		WARNING ("Unexpected answer byte(s).");
 		return DEVICE_STATUS_PROTOCOL;
 	}