diff --git a/dlib/optimization/optimization_solve_qp_using_smo.h b/dlib/optimization/optimization_solve_qp_using_smo.h
index c6655509e..7b737218a 100644
--- a/dlib/optimization/optimization_solve_qp_using_smo.h
+++ b/dlib/optimization/optimization_solve_qp_using_smo.h
@@ -87,6 +87,7 @@ namespace dlib
                      is_col_vector(alpha) &&
                      b.size() == alpha.size() &&
                      b.size() == Q.nr() &&
+                     alpha.size() > 0 &&
                      min(alpha) >= 0 &&
                      eps > 0,
                      "\t void solve_qp_using_smo()"
diff --git a/dlib/optimization/optimization_solve_qp_using_smo_abstract.h b/dlib/optimization/optimization_solve_qp_using_smo_abstract.h
index 2b95274cc..154c19c10 100644
--- a/dlib/optimization/optimization_solve_qp_using_smo_abstract.h
+++ b/dlib/optimization/optimization_solve_qp_using_smo_abstract.h
@@ -27,6 +27,7 @@ namespace dlib
             - is_col_vector(b) == true
             - is_col_vector(alpha) == true
             - b.size() == alpha.size() == Q.nr()
+            - alpha.size() > 0
             - min(alpha) >= 0
             - eps > 0
         ensures