diff --git a/runtime/mercury_minimal_model.c b/runtime/mercury_minimal_model.c
index d9fdaef90..c299511b9 100644
--- a/runtime/mercury_minimal_model.c
+++ b/runtime/mercury_minimal_model.c
@@ -1,7 +1,7 @@
 // vim: ts=4 sw=4 expandtab ft=c
 
 // Copyright (C) 2003-2006 The University of Melbourne.
-// Copyright (C) 2014, 2016, 2018 The Mercury team.
+// Copyright (C) 2014, 2016, 2018, 2025 The Mercury team.
 // This file is distributed under the terms specified in COPYING.LIB.
 
 // This module contains the functions related specifically to the stack copy
@@ -188,9 +188,9 @@ MR_lookup_consumer_debug_addr(MR_Consumer *consumer)
 }
 
 MR_ConsumerDebug *
-MR_lookup_consumer_debug_num(int consumer_index)
+MR_lookup_consumer_debug_num(MR_Unsigned consumer_index)
 {
-    int i;
+    MR_Unsigned i;
 
     for (i = 0; i < MR_consumer_debug_info_next; i++) {
         if (MR_consumer_debug_infos[i].MR_cod_sequence_num == consumer_index) {
@@ -243,7 +243,7 @@ MR_consumer_addr_name(MR_Consumer *consumer)
 }
 
 const char *
-MR_consumer_num_name(int consumer_index)
+MR_consumer_num_name(MR_Unsigned consumer_index)
 {
     MR_ConsumerDebug *consumer_debug;
 
@@ -289,9 +289,9 @@ MR_lookup_subgoal_debug_addr(MR_Subgoal *subgoal)
 }
 
 MR_SubgoalDebug *
-MR_lookup_subgoal_debug_num(int subgoal_index)
+MR_lookup_subgoal_debug_num(MR_Unsigned subgoal_index)
 {
-    int i;
+    MR_Unsigned i;
 
     for (i = 0; i < MR_subgoal_debug_info_next; i++) {
         if (MR_subgoal_debug_infos[i].MR_sgd_sequence_num == subgoal_index) {
diff --git a/runtime/mercury_minimal_model.h b/runtime/mercury_minimal_model.h
index 9ca53cdde..be82f8b28 100644
--- a/runtime/mercury_minimal_model.h
+++ b/runtime/mercury_minimal_model.h
@@ -1,7 +1,7 @@
 // vim: ts=4 sw=4 expandtab ft=c
 
 // Copyright (C) 2003-2006 The University of Melbourne.
-// Copyright (C) 2016, 2018 The Mercury team.
+// Copyright (C) 2016, 2018, 2025 The Mercury team.
 // This file is distributed under the terms specified in COPYING.LIB.
 
 // mercury_minimal_model.h - definitions of some basic stuff used for the stack
@@ -189,14 +189,15 @@ extern  const MR_ProcLayout *MR_subgoal_debug_cur_proc;
 
 extern  void        MR_enter_consumer_debug(MR_Consumer *consumer);
 extern  MR_ConsumerDebug *MR_lookup_consumer_debug_addr(MR_Consumer *consumer);
-extern  MR_ConsumerDebug *MR_lookup_consumer_debug_num(int consumer_index);
+extern  MR_ConsumerDebug *MR_lookup_consumer_debug_num(
+                        MR_Unsigned consumer_index);
 extern  const char  *MR_consumer_debug_name(MR_ConsumerDebug *consumer_dbg);
 extern  const char  *MR_consumer_addr_name(MR_Consumer *consumer);
-extern  const char  *MR_consumer_num_name(int consumer_index);
+extern  const char  *MR_consumer_num_name(MR_Unsigned consumer_index);
 
 extern  void        MR_enter_subgoal_debug(MR_Subgoal *subgoal);
 extern  MR_SubgoalDebug *MR_lookup_subgoal_debug_addr(MR_Subgoal *subgoal);
-extern  MR_SubgoalDebug *MR_lookup_subgoal_debug_num(int subgoal_index);
+extern  MR_SubgoalDebug *MR_lookup_subgoal_debug_num(MR_Unsigned subgoal_index);
 extern  const char  *MR_subgoal_debug_name(MR_SubgoalDebug *subgoal_debug);
 extern  const char  *MR_subgoal_addr_name(MR_Subgoal *subgoal);
 extern  const char  *MR_subgoal_num_name(int subgoal_index);
diff --git a/trace/mercury_trace_cmd_developer.c b/trace/mercury_trace_cmd_developer.c
index 825b39957..db21a3d2b 100644
--- a/trace/mercury_trace_cmd_developer.c
+++ b/trace/mercury_trace_cmd_developer.c
@@ -1,7 +1,7 @@
 // vim: ts=4 sw=4 expandtab ft=c
 
 // Copyright (C) 1998-2008, 2011 The University of Melbourne.
-// Copyright (C) 2013-2016, 2018 The Mercury team.
+// Copyright (C) 2013-2016, 2018, 2025 The Mercury team.
 // This file is distributed under the terms specified in COPYING.LIB.
 
 // This module implements the mdb commands in the "developer" category.
@@ -369,8 +369,8 @@ MR_trace_cmd_subgoal(char **words, int word_count, MR_TraceCmdInfo *cmd,
 #ifdef  MR_USE_MINIMAL_MODEL_STACK_COPY
 
     MR_SubgoalDebug *subgoal_debug;
-    MR_Subgoal  *subgoal;
-    int     n;
+    MR_Subgoal      *subgoal;
+    MR_Unsigned     n;
 
     if (word_count == 2 && MR_trace_is_natural_number(words[1], &n)) {
         MR_trace_init_modules();
@@ -403,7 +403,7 @@ MR_trace_cmd_consumer(char **words, int word_count, MR_TraceCmdInfo *cmd,
 
     MR_ConsumerDebug    *consumer_debug;
     MR_Consumer         *consumer;
-    int                 n;
+    MR_Unsigned         n;
 
     if (word_count == 2 && MR_trace_is_natural_number(words[1], &n)) {
         MR_trace_init_modules();