It's actually fine to call rowm() or colm() with an empty index set.

dlib/matrix/matrix_subexp.h

@@ -386,6 +386,8 @@ namespace dlib
         // the rows matrix must contain integer elements 
         COMPILE_TIME_ASSERT(std::numeric_limits<typename EXP2::type>::is_integer);
 
+#ifdef ENABLE_ASSERTS
+        if (rows.size() != 0) {
             DLIB_ASSERT(0 <= min(rows) && max(rows) < m.nr() && (rows.nr() == 1 || rows.nc() == 1), 
                 "\tconst matrix_exp rowm(const matrix_exp& m, const matrix_exp& rows)"
                 << "\n\tYou have given invalid arguments to this function"
@@ -396,6 +398,8 @@ namespace dlib
                 << "\n\trows.nr():  " << rows.nr()
                 << "\n\trows.nc():  " << rows.nc()
             );
+        }
+#endif // ENABLE_ASSERTS
 
         typedef op_rowm_range<EXP1,EXP2> op;
         return matrix_op<op>(op(m.ref(),rows.ref()));
@@ -541,6 +545,8 @@ namespace dlib
         // the rows matrix must contain integer elements 
         COMPILE_TIME_ASSERT(std::numeric_limits<typename EXP2::type>::is_integer);
 
+#ifdef ENABLE_ASSERTS
+        if (cols.size() != 0) {
             DLIB_ASSERT(0 <= min(cols) && max(cols) < m.nc() && (cols.nr() == 1 || cols.nc() == 1), 
                 "\tconst matrix_exp colm(const matrix_exp& m, const matrix_exp& cols)"
                 << "\n\tYou have given invalid arguments to this function"
@@ -551,6 +557,8 @@ namespace dlib
                 << "\n\tcols.nr():  " << cols.nr()
                 << "\n\tcols.nc():  " << cols.nc()
             );
+        }
+#endif // ENABLE_ASSERTS
 
         typedef op_colm_range<EXP1,EXP2> op;
         return matrix_op<op>(op(m.ref(),cols.ref()));

dlib/matrix/matrix_subexp_abstract.h

@@ -239,8 +239,8 @@ namespace dlib
         requires
             - rows contains integral elements (e.g. int, long)
             - 0 <= min(rows) && max(rows) < m.nr() 
-            - rows.nr() == 1 || rows.nc() == 1
-              (i.e. rows must be a vector)
+            - rows.nr() == 1 || rows.nc() == 1 || rows.size() == 0
+              (i.e. rows must be a vector, or just empty)
         ensures
             - returns a matrix R such that:
                 - R::type == the same type that was in m
@@ -326,8 +326,8 @@ namespace dlib
         requires
             - cols contains integral elements (e.g. int, long)
             - 0 <= min(cols) && max(cols) < m.nc() 
-            - cols.nr() == 1 || cols.nc() == 1
-              (i.e. cols must be a vector)
+            - cols.nr() == 1 || cols.nc() == 1 || cols.size() == 0
+              (i.e. cols must be a vector, or just empty)
         ensures
             - returns a matrix R such that:
                 - R::type == the same type that was in m

dlib/test/matrix.cpp

@@ -1060,6 +1060,8 @@ namespace
             DLIB_TEST(subm(m,range(1,1),range(0,2)) == rowm(m,1));
             DLIB_TEST(subm(m,range(2,2),range(0,2)) == rowm(m,2));
             DLIB_TEST(subm(m,range(3,3),range(0,2)) == rowm(m,3));
+            DLIB_TEST(rowm(m,matrix<long>()).size()==0);
+            DLIB_TEST(colm(m,matrix<long>()).size()==0);
 
             DLIB_TEST(subm(m,0,0,2,2) == subm(m,range(0,1),range(0,1)));
             DLIB_TEST(subm(m,1,1,2,2) == subm(m,range(1,2),range(1,2)));