diff --git a/FPSim2/io/chem.py b/FPSim2/io/chem.py
index a8957f6..085dce3 100644
--- a/FPSim2/io/chem.py
+++ b/FPSim2/io/chem.py
@@ -1,4 +1,4 @@
-from typing import Any, Callable, Iterable as IterableType, Dict, List, Tuple, Union
+from typing import Any, Callable, Iterable as IterableType, Dict, Tuple, Union
 from FPSim2.FPSim2lib.utils import BitStrToIntList, PyPopcount
 from collections.abc import Iterable
 from rdkit.Chem import rdMolDescriptors
diff --git a/FPSim2/src/utils.cpp b/FPSim2/src/utils.cpp
index 457cab1..ae1fa92 100644
--- a/FPSim2/src/utils.cpp
+++ b/FPSim2/src/utils.cpp
@@ -23,8 +23,13 @@ namespace utils {
 
     py::list BitStrToIntList(const std::string &bit_string) {
         py::list efp;
-        for (size_t i = 0; i < bit_string.length(); i += 64) {
-            efp.append(std::stoull(bit_string.substr(i, 64), 0, 2));
+        size_t len = bit_string.length();
+        for (size_t i = 0; i < len; i += 64) {
+            uint64_t value = 0;
+            for (size_t j = 0; j < 64 && (i + j) < len; ++j) {
+                value = (value << 1) | (bit_string[i + j] - '0');
+            }
+            efp.append(value);
         }
         return efp;
     }