diff --git a/internal/ceres/residual_block_utils.cc b/internal/ceres/residual_block_utils.cc
index ff18e21..4d88a9f 100644
--- a/internal/ceres/residual_block_utils.cc
+++ b/internal/ceres/residual_block_utils.cc
@@ -63,6 +63,7 @@
 
 // Utility routine to print an array of doubles to a string. If the
 // array pointer is NULL, it is treated as an array of zeros.
+namespace {
 void AppendArrayToString(const int size, const double* x, string* result) {
   for (int i = 0; i < size; ++i) {
     if (x == NULL) {
@@ -76,6 +77,7 @@
     }
   }
 }
+}  // namespace
 
 string EvaluationToString(const ResidualBlock& block,
                           double const* const* parameters,