diff --git a/src/sna/sna_display.c b/src/sna/sna_display.c
index 889019c8..8982d6a4 100644
--- a/src/sna/sna_display.c
+++ b/src/sna/sna_display.c
@@ -1919,6 +1919,7 @@ sna_output_detect(xf86OutputPtr output)
 			sna_output->num_modes = min(old_count, sna_output->num_modes);
 			break;
 		}
+		VG(VALGRIND_MAKE_MEM_DEFINED(sna_output->modes, sizeof(*sna_output->modes)*sna_output->num_modes));
 	}
 
 	DBG(("%s(%s): found %d modes, connection status=%d\n",