diff --git a/src/lib.rs b/src/lib.rs
index 9356db2..b6fca92 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -688,23 +688,78 @@ fn test_solve_random10() {
 fn test_invalid() {
     use std::f32;
     const N: usize = 3;
-
     let c = vec![
+        // row 0
         250.0,
-        400.0,
+        400.0, // (a)
         350.0,
-        400.0,
+        // row 1
+        400.0, // (b)
         600.0,
         f32::INFINITY,
+        // row 2
         200.0,
         400.0,
+        250.0, // (c)
+    ];
+
+    let mut weights: WeightMatrix<f32> = WeightMatrix::from_row_vec(N, c.clone());
+    let matching = solve_assignment(&mut weights).unwrap();
+    assert_eq!(N, matching.len());
+    assert_eq!(1050.0, calc_cost(0.0, &c[..], &matching[..], N));
+    assert_eq!(vec![pos(0, 1), pos(1, 0), pos(2, 2)], matching);
+}
+
+#[test]
+fn test_should_be_unsolvable_if_first_cell_of_each_row_is_invalid() {
+    use std::f32;
+    const N: usize = 3;
+
+    let c = vec![
+        // row 0
+        f32::INFINITY,
+        400.0,
+        350.0,
+        // row 1
+        f32::INFINITY,
+        400.0,
+        600.0,
+        // row 2
+        f32::INFINITY,
+        400.0,
         250.0,
     ];
 
-    let mut weights: WeightMatrix<f32> = WeightMatrix::from_row_vec(N, c);
-    let matching = solve_assignment(&mut weights).unwrap();
+    let mut weights: WeightMatrix<f32> = WeightMatrix::from_row_vec(N, c.clone());
+    let res = solve_assignment(&mut weights);
+    assert_eq!(Err(Error::MatrixNotSolvable), res);
+}
 
-    assert_eq!(vec![pos(0, 1), pos(1, 0), pos(2, 2)], matching);
+#[test]
+fn test_invalid2() {
+    use std::f32;
+    const N: usize = 3;
+
+    let c = vec![
+        // row 0
+        f32::INFINITY,
+        400.0,
+        350.0, // (a)
+        // row 1
+        f32::INFINITY,
+        400.0, // (b)
+        600.0,
+        // row 2
+        400.0, // (c)
+        250.0,
+        f32::INFINITY,
+    ];
+
+    let mut weights: WeightMatrix<f32> = WeightMatrix::from_row_vec(N, c.clone());
+    let matching = solve_assignment(&mut weights).unwrap();
+    assert_eq!(N, matching.len());
+    assert_eq!(1150.0, calc_cost(0.0, &c[..], &matching[..], N));
+    assert_eq!(vec![pos(0, 2), pos(1, 1), pos(2, 0)], matching);
 }
 
 #[test]
diff --git a/src/weight_matrix.rs b/src/weight_matrix.rs
index 1580b2e..ff58641 100644
--- a/src/weight_matrix.rs
+++ b/src/weight_matrix.rs
@@ -42,8 +42,13 @@ impl<T: WeightNum> Weights for WeightMatrix<T> {
     }
 
     fn is_solvable(&self) -> bool {
-        for row in 0..self.n() {
-            if self.c.row(row).iter().all(|c| !c.is_valid()) {
+        for row in self.c.genrows() {
+            if row.iter().all(|c| !c.is_valid()) {
+                return false;
+            }
+        }
+        for column in self.c.gencolumns() {
+            if column.iter().all(|c| !c.is_valid()) {
                 return false;
             }
         }
@@ -67,14 +72,13 @@ impl<T: WeightNum> WeightMatrix<T> {
 
     /// Return the minimum element of row `row`.
     fn min_of_row(&self, row: usize) -> T {
-        let row_slice = self.c.row(row);
-        let mut min = row_slice[0];
-        for &val in row_slice.iter().skip(1) {
-            if val.is_valid() && val < min {
-                min = val;
-            }
-        }
-        min
+        let row_iter = self.c.row(row);
+        let mut valid_iter = row_iter.iter().filter(|cost| cost.is_valid()).cloned();
+        let first_min = valid_iter.next().unwrap();
+        valid_iter.fold(
+            first_min,
+            |total_min, val| if val < total_min { val } else { total_min },
+        )
     }
 
     // Subtract `val` from every element in row `row`.