diff --git a/internal/ceres/dense_sparse_matrix.h b/internal/ceres/dense_sparse_matrix.h
index 5ce29ee..416c214 100644
--- a/internal/ceres/dense_sparse_matrix.h
+++ b/internal/ceres/dense_sparse_matrix.h
@@ -70,6 +70,7 @@
 #ifndef CERES_DONT_HAVE_PROTOCOL_BUFFERS
   virtual void ToProto(SparseMatrixProto* proto) const;
 #endif
+  virtual void ToTextFile(FILE* file) const;
   virtual int num_rows() const;
   virtual int num_cols() const;
   virtual int num_nonzeros() const;