diff --git a/internal/ceres/polynomial.cc b/internal/ceres/polynomial.cc
index 3238b89..feec884 100644
--- a/internal/ceres/polynomial.cc
+++ b/internal/ceres/polynomial.cc
@@ -37,6 +37,7 @@
 
 #include "Eigen/Dense"
 #include "ceres/internal/port.h"
+#include "ceres/stringprintf.h"
 #include "glog/logging.h"
 
 namespace ceres {
@@ -255,6 +256,12 @@
   }
 }
 
+string FunctionSample::ToDebugString() const {
+  return StringPrintf("[x: %.8e, value: %.8e, gradient: %.8e, "
+                      "value_is_valid: %d, gradient_is_valid: %d]",
+                      x, value, gradient, value_is_valid, gradient_is_valid);
+}
+
 Vector FindInterpolatingPolynomial(const vector<FunctionSample>& samples) {
   const int num_samples = samples.size();
   int num_constraints = 0;