diff --git a/backend/drm/drm.c b/backend/drm/drm.c
index 0acb0324..332abfe5 100644
--- a/backend/drm/drm.c
+++ b/backend/drm/drm.c
@@ -550,11 +550,18 @@ bool wlr_drm_connector_add_mode(struct wlr_output *output,
 		const drmModeModeInfo *modeinfo) {
 	struct wlr_drm_connector *conn = get_drm_connector_from_output(output);
 
-	assert(modeinfo);
 	if (modeinfo->type != DRM_MODE_TYPE_USERDEF) {
 		return false;
 	}
 
+	struct wlr_output_mode *wlr_mode;
+	wl_list_for_each(wlr_mode, &conn->output.modes, link) {
+		struct wlr_drm_mode *mode = (struct wlr_drm_mode *)wlr_mode;
+		if (memcmp(&mode->drm_mode, modeinfo, sizeof(*modeinfo)) == 0) {
+			return true;
+		}
+	}
+
 	struct wlr_drm_mode *mode = calloc(1, sizeof(*mode));
 	if (!mode) {
 		return false;
@@ -563,7 +570,7 @@ bool wlr_drm_connector_add_mode(struct wlr_output *output,
 
 	mode->wlr_mode.width = mode->drm_mode.hdisplay;
 	mode->wlr_mode.height = mode->drm_mode.vdisplay;
-	mode->wlr_mode.refresh = mode->drm_mode.vrefresh;
+	mode->wlr_mode.refresh = calculate_refresh_rate(modeinfo);
 
 	wlr_log(WLR_INFO, "Registered custom mode "
 			"%"PRId32"x%"PRId32"@%"PRId32,
diff --git a/backend/drm/util.c b/backend/drm/util.c
index 6b2c0981..6a8b346a 100644
--- a/backend/drm/util.c
+++ b/backend/drm/util.c
@@ -6,7 +6,7 @@
 #include <wlr/util/log.h>
 #include "backend/drm/util.h"
 
-int32_t calculate_refresh_rate(drmModeModeInfo *mode) {
+int32_t calculate_refresh_rate(const drmModeModeInfo *mode) {
 	int32_t refresh = (mode->clock * 1000000LL / mode->htotal +
 		mode->vtotal / 2) / mode->vtotal;
 
diff --git a/include/backend/drm/util.h b/include/backend/drm/util.h
index 0b55182b..e159d716 100644
--- a/include/backend/drm/util.h
+++ b/include/backend/drm/util.h
@@ -7,7 +7,7 @@
 #include <xf86drmMode.h>
 
 // Calculates a more accurate refresh rate (mHz) than what mode itself provides
-int32_t calculate_refresh_rate(drmModeModeInfo *mode);
+int32_t calculate_refresh_rate(const drmModeModeInfo *mode);
 // Populates the make/model/phys_{width,height} of output from the edid data
 void parse_edid(struct wlr_output *restrict output, size_t len,
 	const uint8_t *data);