diff --git a/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c b/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c
index bd61e20770a5..719da3610310 100644
--- a/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_dual_mode_helper.c
@@ -160,11 +160,11 @@ EXPORT_SYMBOL(drm_dp_dual_mode_write);
 
 static bool is_hdmi_adaptor(const char hdmi_id[DP_DUAL_MODE_HDMI_ID_LEN])
 {
-	static const char dp_dual_mode_hdmi_id[DP_DUAL_MODE_HDMI_ID_LEN] =
+	static const char dp_dual_mode_hdmi_id[DP_DUAL_MODE_HDMI_ID_LEN + 1] =
 		"DP-HDMI ADAPTOR\x04";
 
 	return memcmp(hdmi_id, dp_dual_mode_hdmi_id,
-		      sizeof(dp_dual_mode_hdmi_id)) == 0;
+		      DP_DUAL_MODE_HDMI_ID_LEN) == 0;
 }
 
 static bool is_type1_adaptor(uint8_t adaptor_id)