diff --git a/Solver/STensor33.h b/Solver/STensor33.h
index 7795c866b6ea452e6e9936355282185d052b5c11..f14ee8ac9071f53def48a92c79fc12ba68c2816f 100644
--- a/Solver/STensor33.h
+++ b/Solver/STensor33.h
@@ -49,6 +49,11 @@ class STensor33 {
     for (int i = 0; i < 27; i++) _val[i] += other._val[i];
     return *this;
   }
+  STensor33& operator -= (const STensor33 &other)
+  {
+    for (int i = 0; i < 27; i++) _val[i] -= other._val[i];
+    return *this;
+  }
   STensor33& operator *= (const double &other)
   {
     for (int i = 0; i < 27; i++) _val[i] *= other;