diff --git a/dlib/optimization/optimization_solve_qp_using_smo.h b/dlib/optimization/optimization_solve_qp_using_smo.h
index b9cb8dcef..a44875046 100644
--- a/dlib/optimization/optimization_solve_qp_using_smo.h
+++ b/dlib/optimization/optimization_solve_qp_using_smo.h
@@ -632,6 +632,7 @@ namespace dlib
         for (auto& Q : Q_blocks)
         {
             DLIB_CASSERT(Q.nr() == Q.nc(), "All the matrices in Q_blocks have the same dimensions.");
+            DLIB_CASSERT(Q.size() > 0, "All the matrices in Q_blocks must be non-empty and have the same dimensions.");
             DLIB_CASSERT(Q.nr() == Q_blocks[0].nr() && Q.nc() == Q_blocks[0].nc(), "All the matrices in Q_blocks have the same dimensions.");
         }
 #ifdef ENABLE_ASSERTS
diff --git a/dlib/optimization/optimization_solve_qp_using_smo_abstract.h b/dlib/optimization/optimization_solve_qp_using_smo_abstract.h
index 6fe2be8bf..5e7d5ec3f 100644
--- a/dlib/optimization/optimization_solve_qp_using_smo_abstract.h
+++ b/dlib/optimization/optimization_solve_qp_using_smo_abstract.h
@@ -183,8 +183,8 @@ namespace dlib
         requires
             - Q_blocks.size() > 0
             - Q_blocks.size() == bs.size() == alphas.size() == lowers.size() == uppers.size()
-            - All the matrices in Q_blocks have the same dimensions.  Moreover, they are square
-              matrices.
+            - All the matrices in Q_blocks have the same dimensions.  Moreover, they are
+              non-empty square matrices.
             - All the matrices in bs, Q_offdiag, alphas, lowers, and uppers have the same
               dimensions.  Moreover, they are all column vectors.
             - Q_blocks[0].nr() == alphas[0].size()