diff --git a/arrays.scad b/arrays.scad
index 8f44fe8..37fee58 100644
--- a/arrays.scad
+++ b/arrays.scad
@@ -1224,6 +1224,7 @@ function block_matrix(M) =
 //   its diagonal.  The off diagonal entries are set to offdiag,
 //   which is zero by default. 
 function diagonal_matrix(diag,offdiag=0) =
+  assert(is_list(diag) && len(diag)>0)
   [for(i=[0:1:len(diag)-1]) [for(j=[0:len(diag)-1]) i==j?diag[i] : offdiag]];
 
 
@@ -1237,6 +1238,8 @@ function diagonal_matrix(diag,offdiag=0) =
 function submatrix_set(M,A,m=0,n=0) =
     assert(is_list(M))
     assert(is_list(A))
+    assert(is_int(m))
+    assert(is_int(n))
     let( badrows = [for(i=idx(A)) if (!is_list(A[i])) i])
     assert(badrows==[], str("Input submatrix malformed rows: ",badrows))
     [for(i=[0:1:len(M)-1])
diff --git a/math.scad b/math.scad
index 599a0c4..934d332 100644
--- a/math.scad
+++ b/math.scad
@@ -894,7 +894,8 @@ function is_matrix(A,m,n,square=false) =
 //    squares of all of the entries of the matrix.  On vectors it is the same as the usual 2-norm.
 //    This is an easily computed norm that is convenient for comparing two matrices.  
 function norm_fro(A) =
-    sqrt(sum([for(entry=A) sum_of_squares(entry)]));
+    assert(is_matrix(A) || is_vector(A))
+    norm(flatten(A));
 
 
 // Section: Comparisons and Logic