diff --git a/modules/core/test/test_mat.cpp b/modules/core/test/test_mat.cpp
index 2344df28a5f3deb2309a969c0dcacdc576ebe56d..e2de3249b68260231305c6d16a828b7d708d7e94 100644
--- a/modules/core/test/test_mat.cpp
+++ b/modules/core/test/test_mat.cpp
@@ -1472,3 +1472,14 @@ TEST(Core_Mat_vector, copyTo_roi_row)
     EXPECT_EQ(4, (int)dst2[3]);
     EXPECT_EQ(5, (int)dst2[4]);
 }
+
+TEST(Mat, regression_5991)
+{
+    int sz[] = {2,3,2};
+    Mat mat(3, sz, CV_32F, Scalar(1));
+    ASSERT_NO_THROW(mat.convertTo(mat, CV_8U));
+    EXPECT_EQ(sz[0], mat.size[0]);
+    EXPECT_EQ(sz[1], mat.size[1]);
+    EXPECT_EQ(sz[2], mat.size[2]);
+    EXPECT_EQ(0, cvtest::norm(mat, Mat(3, sz, CV_8U, Scalar(1)), NORM_INF));
+}
diff --git a/modules/core/test/test_umat.cpp b/modules/core/test/test_umat.cpp
index a8641d098089e3310ffe5fb48f811691568b0cc3..09c59b60d24772d87131983c68bc7bae6478b9c6 100644
--- a/modules/core/test/test_umat.cpp
+++ b/modules/core/test/test_umat.cpp
@@ -1343,4 +1343,15 @@ TEST(UMat, testWrongLifetime_Mat)
     }
 }
 
+TEST(UMat, DISABLED_regression_5991)
+{
+    int sz[] = {2,3,2};
+    UMat mat(3, sz, CV_32F, Scalar(1));
+    ASSERT_NO_THROW(mat.convertTo(mat, CV_8U));
+    EXPECT_EQ(sz[0], mat.size[0]);
+    EXPECT_EQ(sz[1], mat.size[1]);
+    EXPECT_EQ(sz[2], mat.size[2]);
+    EXPECT_EQ(0, cvtest::norm(mat.getMat(ACCESS_READ), Mat(3, sz, CV_8U, Scalar(1)), NORM_INF));
+}
+
 } } // namespace cvtest::ocl