diff --git a/jbmc/src/java_bytecode/java_string_library_preprocess.cpp b/jbmc/src/java_bytecode/java_string_library_preprocess.cpp
index f892c7400a5..822fcd46a97 100644
--- a/jbmc/src/java_bytecode/java_string_library_preprocess.cpp
+++ b/jbmc/src/java_bytecode/java_string_library_preprocess.cpp
@@ -1342,7 +1342,7 @@ exprt java_string_library_preprocesst::get_primitive_value_of_object(
 /// \return An expression representing the object at position `index` of `argv`.
 exprt java_string_library_preprocesst::get_object_at_index(
   const exprt &argv,
-  int index)
+  std::size_t index)
 {
   dereference_exprt deref_objs(argv, argv.type().subtype());
   pointer_typet empty_pointer=pointer_type(empty_typet());
@@ -1390,7 +1390,7 @@ exprt java_string_library_preprocesst::get_object_at_index(
 ///         values of the argument at position `index` of `argv`.
 exprt java_string_library_preprocesst::make_argument_for_format(
   const exprt &argv,
-  int index,
+  std::size_t index,
   const struct_typet &structured_type,
   const source_locationt &loc,
   const irep_idt &function_id,
diff --git a/jbmc/src/java_bytecode/java_string_library_preprocess.h b/jbmc/src/java_bytecode/java_string_library_preprocess.h
index 84317ed9163..382dc1b5d9d 100644
--- a/jbmc/src/java_bytecode/java_string_library_preprocess.h
+++ b/jbmc/src/java_bytecode/java_string_library_preprocess.h
@@ -337,7 +337,7 @@ class java_string_library_preprocesst:public messaget
 
   exprt make_argument_for_format(
     const exprt &argv,
-    int index,
+    std::size_t index,
     const struct_typet &structured_type,
     const source_locationt &loc,
     const irep_idt &function_id,
@@ -351,7 +351,7 @@ class java_string_library_preprocesst:public messaget
     symbol_table_baset &symbol_table,
     code_blockt &code);
 
-  exprt get_object_at_index(const exprt &argv, int index);
+  exprt get_object_at_index(const exprt &argv, std::size_t index);
 
   codet make_init_from_array_code(
     const code_typet &type,