diff --git a/src/weight_matrix.rs b/src/weight_matrix.rs
index 233ec85..1580b2e 100644
--- a/src/weight_matrix.rs
+++ b/src/weight_matrix.rs
@@ -31,14 +31,14 @@ impl<T: WeightNum> Weights for WeightMatrix<T> {
     fn add_row(&mut self, row: usize, val: T) {
         self.c
             .row_mut(row)
-            .mapv_inplace(|cur| if cur.is_valid() { cur + val } else { cur });
+            .mapv_inplace(|cur| cur.add_if_valid(val));
     }
 
     // Subtract `val` from every element in column `col`.
     fn sub_column(&mut self, col: usize, val: T) {
         self.c
             .column_mut(col)
-            .mapv_inplace(|cur| if cur.is_valid() { cur - val } else { cur });
+            .mapv_inplace(|cur| cur.sub_if_valid(val));
     }
 
     fn is_solvable(&self) -> bool {
@@ -81,7 +81,7 @@ impl<T: WeightNum> WeightMatrix<T> {
     fn sub_row(&mut self, row: usize, val: T) {
         self.c
             .row_mut(row)
-            .mapv_inplace(|cur| if cur.is_valid() { cur - val } else { cur });
+            .mapv_inplace(|cur| cur.sub_if_valid(val));
     }
 
     pub fn as_slice(&self) -> &[T] {
diff --git a/src/weight_num.rs b/src/weight_num.rs
index 57b37ef..2faeef5 100644
--- a/src/weight_num.rs
+++ b/src/weight_num.rs
@@ -6,6 +6,20 @@ pub trait WeightNum: PartialOrd + Copy + Sub<Output = Self> + Add<Output = Self>
     fn is_valid(&self) -> bool {
         true
     }
+    fn add_if_valid(self, other: Self) -> Self {
+        if self.is_valid() {
+            self + other
+        } else {
+            self
+        }
+    }
+    fn sub_if_valid(self, other: Self) -> Self {
+        if self.is_valid() {
+            self - other
+        } else {
+            self
+        }
+    }
 }
 
 impl WeightNum for usize {