diff --git a/src/scopflow/solver/hiop/scopflow_hiop.cpp b/src/scopflow/solver/hiop/scopflow_hiop.cpp
index 1aee91be..7dcfa19a 100644
--- a/src/scopflow/solver/hiop/scopflow_hiop.cpp
+++ b/src/scopflow/solver/hiop/scopflow_hiop.cpp
@@ -155,7 +155,7 @@ extern PetscErrorCode SCOPFLOWUpdateOPFLOWVariableBounds(OPFLOW, Vec, Vec,
 /* Note: x only holds the coupled variables which, in this case, are the
    generator real power variables for the base-case
 */
-bool SCOPFLOWHIOPInterface::eval_f_rterm(size_t idx, const int &n,
+bool SCOPFLOWHIOPInterface::eval_f_rterm(hiop::size_type idx, const int &n,
                                          const double *x, double &rval) {
   PetscErrorCode ierr;
   OPFLOW opflow0; /* base case OPFLOW */
@@ -245,7 +245,8 @@ bool SCOPFLOWHIOPInterface::eval_f_rterm(size_t idx, const int &n,
   return true;
 }
 
-bool SCOPFLOWHIOPInterface::eval_grad_rterm(size_t idx, const int &n, double *x,
+bool SCOPFLOWHIOPInterface::eval_grad_rterm(hiop::size_type idx, const int &n,
+                                            double *x,
                                             hiop::hiopVector &gradvec) {
   (void)idx;
   (void)n;
@@ -310,12 +311,12 @@ bool SCOPFLOWHIOPInterface::eval_grad_rterm(size_t idx, const int &n, double *x,
   return true;
 }
 
-size_t SCOPFLOWHIOPInterface::get_num_rterms() const {
-  return scopflow->Nc - 1;
+hiop::size_type SCOPFLOWHIOPInterface::get_num_rterms() const {
+  return (hiop::size_type)scopflow->Nc - 1;
 }
 
-size_t SCOPFLOWHIOPInterface::get_num_vars() const {
-  return scopflow->opflow0->nx;
+hiop::size_type SCOPFLOWHIOPInterface::get_num_vars() const {
+  return (hiop::size_type)scopflow->opflow0->nx;
 }
 
 void SCOPFLOWHIOPInterface::get_solution(double *x) const {
diff --git a/src/scopflow/solver/hiop/scopflow_hiop.h b/src/scopflow/solver/hiop/scopflow_hiop.h
index b01f50b4..834cb2ff 100644
--- a/src/scopflow/solver/hiop/scopflow_hiop.h
+++ b/src/scopflow/solver/hiop/scopflow_hiop.h
@@ -23,13 +23,14 @@ class SCOPFLOWHIOPInterface : public hiop::hiopInterfacePriDecProblem {
                                      const double *hess,
                                      const char *master_options_file);
 
-  bool eval_f_rterm(size_t idx, const int &n, const double *x, double &rval);
-  bool eval_grad_rterm(size_t idx, const int &n, double *x,
+  bool eval_f_rterm(hiop::size_type idx, const int &n, const double *x,
+                    double &rval);
+  bool eval_grad_rterm(hiop::size_type idx, const int &n, double *x,
                        hiop::hiopVector &grad);
 
-  size_t get_num_rterms() const;
+  hiop::size_type get_num_rterms() const;
 
-  size_t get_num_vars() const;
+  hiop::size_type get_num_vars() const;
 
   void get_solution(double *x) const;
 
diff --git a/src/sopflow/solver/hiop/sopflow_hiop.cpp b/src/sopflow/solver/hiop/sopflow_hiop.cpp
index c5cd759a..8b933ca5 100644
--- a/src/sopflow/solver/hiop/sopflow_hiop.cpp
+++ b/src/sopflow/solver/hiop/sopflow_hiop.cpp
@@ -164,7 +164,7 @@ extern PetscErrorCode SOPFLOWUpdateOPFLOWVariableBounds(OPFLOW, Vec, Vec,
 /* Note: x only holds the coupled variables which, in this case, are the
    generator real power variables for the base-case
 */
-bool SOPFLOWHIOPInterface::eval_f_rterm(size_t idx, const int &n,
+bool SOPFLOWHIOPInterface::eval_f_rterm(hiop::size_type idx, const int &n,
                                         const double *x, double &rval) {
   (void)n;
   PetscErrorCode ierr;
@@ -283,7 +283,8 @@ bool SOPFLOWHIOPInterface::eval_f_rterm(size_t idx, const int &n,
   return true;
 }
 
-bool SOPFLOWHIOPInterface::eval_grad_rterm(size_t idx, const int &n, double *x,
+bool SOPFLOWHIOPInterface::eval_grad_rterm(hiop::size_type idx, const int &n,
+                                           double *x,
                                            hiop::hiopVector &gradvec) {
   (void)idx;
   (void)n;
@@ -349,11 +350,11 @@ bool SOPFLOWHIOPInterface::eval_grad_rterm(size_t idx, const int &n, double *x,
   return true;
 }
 
-size_t SOPFLOWHIOPInterface::get_num_rterms() const {
+hiop::size_type SOPFLOWHIOPInterface::get_num_rterms() const {
   return (sopflow->Ns * sopflow->Nc) - 1;
 }
 
-size_t SOPFLOWHIOPInterface::get_num_vars() const {
+hiop::size_type SOPFLOWHIOPInterface::get_num_vars() const {
   return sopflow->opflow0->nx;
 }
 
diff --git a/src/sopflow/solver/hiop/sopflow_hiop.h b/src/sopflow/solver/hiop/sopflow_hiop.h
index b3f32d69..0aab4ffd 100644
--- a/src/sopflow/solver/hiop/sopflow_hiop.h
+++ b/src/sopflow/solver/hiop/sopflow_hiop.h
@@ -24,13 +24,14 @@ class SOPFLOWHIOPInterface : public hiop::hiopInterfacePriDecProblem {
                                      const double *hess,
                                      const char *master_options_file);
 
-  bool eval_f_rterm(size_t idx, const int &n, const double *x, double &rval);
-  bool eval_grad_rterm(size_t idx, const int &n, double *x,
+  bool eval_f_rterm(hiop::size_type idx, const int &n, const double *x,
+                    double &rval);
+  bool eval_grad_rterm(hiop::size_type idx, const int &n, double *x,
                        hiop::hiopVector &grad);
 
-  size_t get_num_rterms() const;
+  hiop::size_type get_num_rterms() const;
 
-  size_t get_num_vars() const;
+  hiop::size_type get_num_vars() const;
 
   void get_solution(double *x) const;