diff --git a/apyds/ds.cc b/apyds/ds.cc
index 142914f..b1b80bf 100644
--- a/apyds/ds.cc
+++ b/apyds/ds.cc
@@ -18,7 +18,7 @@ class Iterator {
             iterator = std::make_unique<iterator_t>(generator.begin());
             initialized = true;
         }
-        if (*iterator == nullptr) {
+        if (*iterator == generator.end()) {
             return nullptr;
         }
         ds::rule_t* result = **iterator;
diff --git a/atsds/ds.cc b/atsds/ds.cc
index 03bbe29..f94f84c 100644
--- a/atsds/ds.cc
+++ b/atsds/ds.cc
@@ -17,7 +17,7 @@ class Iterator {
             iterator = std::make_unique<iterator_t>(generator.begin());
             initialized = true;
         }
-        if (*iterator == nullptr) {
+        if (*iterator == generator.end()) {
             return nullptr;
         }
         ds::rule_t* result = **iterator;