diff --git a/apriltag.c b/apriltag.c
index ec4f0224..e483fb8f 100644
--- a/apriltag.c
+++ b/apriltag.c
@@ -445,6 +445,8 @@ static matd_t* homography_compute2(double c[4][4]) {
             }
         }
 
+        assert(max_val_idx != -1);
+
         if (max_val < epsilon) {
             debug_print("WRN: Matrix is singular.\n");
             return NULL;