diff --git a/src/ftrl.c b/src/ftrl.c
index 8405730f..1dd02166 100644
--- a/src/ftrl.c
+++ b/src/ftrl.c
@@ -208,7 +208,7 @@ double ftrl_reg_cost(ftrl_trainer_t *self, double_matrix_t *theta, uint32_array
         if (row_idx >= i_start) {
             double *theta_i = double_matrix_get_row(theta, i);
 
-            l2_cost += double_array_l2_norm(theta_i, n);
+            l2_cost += double_array_sum_sq(theta_i, n);
             l1_cost += double_array_l1_norm(theta_i, n);
         }
     }
diff --git a/src/stochastic_gradient_descent.c b/src/stochastic_gradient_descent.c
index 19bca2a7..16e5f153 100644
--- a/src/stochastic_gradient_descent.c
+++ b/src/stochastic_gradient_descent.c
@@ -295,7 +295,7 @@ double stochastic_gradient_descent_reg_cost(sgd_trainer_t *self, uint32_array *u
         double *theta_i = double_matrix_get_row(theta, row);
 
         if (reg_type == REGULARIZATION_L2 && row >= i_start) {
-            cost += double_array_l2_norm(theta_i, n);
+            cost += double_array_sum_sq(theta_i, n);
         } else if (reg_type == REGULARIZATION_L1 && row >= i_start) {
             cost += double_array_l1_norm(theta_i, n);
         }
diff --git a/src/vector_math.h b/src/vector_math.h
index 14aaf7a1..ff9c017c 100644
--- a/src/vector_math.h
+++ b/src/vector_math.h
@@ -210,6 +210,14 @@
         return sqrt((double)result);                                                                    \
     }                                                                                                   \
                                                                                                         \
+    static inline unsigned_type name##_sum_sq(type *array, size_t n) {                                  \
+        unsigned_type result = 0;                                                                       \
+        for (size_t i = 0; i < n; i++) {                                                                \
+            result += array[i] * array[i];                                                              \
+        }                                                                                               \
+        return result;                                                                                  \
+    }                                                                                                   \
+                                                                                                        \
     static inline double name##_mean(type *array, size_t n) {                                           \
         unsigned_type sum = name##_sum(array, n);                                                       \
         return (double)sum / n;                                                                         \