diff --git a/src/tensor/algstrct.cxx b/src/tensor/algstrct.cxx
index ce9ee570..59a8315b 100644
--- a/src/tensor/algstrct.cxx
+++ b/src/tensor/algstrct.cxx
@@ -891,7 +891,7 @@ namespace CTF_int {
     sr=sr_; ptr=ptr_;
   }
 
-  ConstPairIterator ConstPairIterator::operator[](int n) const { 
+  ConstPairIterator ConstPairIterator::operator[](int64_t n) const { 
     return ConstPairIterator(sr,ptr+sr->pair_size()*n);
   }
 
@@ -916,7 +916,7 @@ namespace CTF_int {
     ptr=ptr_;
   }
 
-  PairIterator PairIterator::operator[](int n) const { 
+  PairIterator PairIterator::operator[](int64_t n) const { 
     return PairIterator(sr,ptr+sr->pair_size()*n);
   }
 
diff --git a/src/tensor/algstrct.h b/src/tensor/algstrct.h
index 6672b5d6..c901a99d 100644
--- a/src/tensor/algstrct.h
+++ b/src/tensor/algstrct.h
@@ -462,7 +462,7 @@ namespace CTF_int {
       ConstPairIterator(algstrct const * sr_, char const * ptr_);
 
       /** \brief indexing moves by \param[in] n pairs */
-      ConstPairIterator operator[](int n) const;
+      ConstPairIterator operator[](int64_t n) const;
 
       /** \brief returns key of pair at head of ptr */
       int64_t k() const;
@@ -513,7 +513,7 @@ namespace CTF_int {
       PairIterator(algstrct const * sr_, char * ptr_);
 
       /** \brief indexing moves by \param[in] n pairs */
-      PairIterator operator[](int n) const;
+      PairIterator operator[](int64_t n) const;
 
       /** \brief returns key of pair at head of ptr */
       int64_t k() const;