diff --git a/src/solvers/refinement/string_refinement_util.cpp b/src/solvers/refinement/string_refinement_util.cpp
index 46cb5178389..c9071509452 100644
--- a/src/solvers/refinement/string_refinement_util.cpp
+++ b/src/solvers/refinement/string_refinement_util.cpp
@@ -292,17 +292,19 @@ std::vector<mp_integer> string_concatenation_builtin_functiont::eval(
   const std::vector<mp_integer> &input2_value,
   const std::vector<mp_integer> &args_value) const
 {
-  const std::size_t start_index =
-    args_value.size() > 0 && args_value[0] > 0 ? args_value[0].to_ulong() : 0;
-  const std::size_t end_index = args_value.size() > 1 && args_value[1] > 0
-                                  ? args_value[1].to_ulong()
-                                  : input2_value.size();
+  const auto start_index =
+    args_value.size() > 0 && args_value[0] > 0 ? args_value[0] : mp_integer(0);
+  const mp_integer input2_size(input2_value.size());
+  const auto end_index =
+    args_value.size() > 1
+      ? std::max(std::min(args_value[1], input2_size), start_index)
+      : input2_size;
 
   std::vector<mp_integer> result(input1_value);
   result.insert(
     result.end(),
-    input2_value.begin() + start_index,
-    input2_value.begin() + end_index);
+    input2_value.begin() + numeric_cast_v<std::size_t>(start_index),
+    input2_value.begin() + numeric_cast_v<std::size_t>(end_index));
   return result;
 }