diff --git a/builtin/checkout.c b/builtin/checkout.c
index b49b582071..5cebe170fc 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -526,6 +526,7 @@ static int merge_working_tree(const struct checkout_opts *opts,
 		init_tree_desc(&trees[1], tree->buffer, tree->size);
 
 		ret = unpack_trees(2, trees, &topts);
+		clear_unpack_trees_porcelain(&topts);
 		if (ret == -1) {
 			/*
 			 * Unpack couldn't do a trivial merge; either
diff --git a/merge-recursive.c b/merge-recursive.c
index ddb0fa7369..338f63a952 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -382,6 +382,7 @@ static int unpack_trees_start(struct merge_options *o,
 static void unpack_trees_finish(struct merge_options *o)
 {
 	discard_index(&o->orig_index);
+	clear_unpack_trees_porcelain(&o->unpack_opts);
 }
 
 struct tree *write_tree_from_memory(struct merge_options *o)
diff --git a/merge.c b/merge.c
index f123658e58..b433291d0c 100644
--- a/merge.c
+++ b/merge.c
@@ -130,8 +130,11 @@ int checkout_fast_forward(const struct object_id *head,
 
 	if (unpack_trees(nr_trees, t, &opts)) {
 		rollback_lock_file(&lock_file);
+		clear_unpack_trees_porcelain(&opts);
 		return -1;
 	}
+	clear_unpack_trees_porcelain(&opts);
+
 	if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
 		return error(_("unable to write new index file"));
 	return 0;
diff --git a/unpack-trees.c b/unpack-trees.c
index 79fd97074e..73a6dc1701 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1,5 +1,6 @@
 #define NO_THE_INDEX_COMPATIBILITY_MACROS
 #include "cache.h"
+#include "argv-array.h"
 #include "repository.h"
 #include "config.h"
 #include "dir.h"
@@ -103,6 +104,8 @@ void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
 	const char **msgs = opts->msgs;
 	const char *msg;
 
+	argv_array_init(&opts->msgs_to_free);
+
 	if (!strcmp(cmd, "checkout"))
 		msg = advice_commit_before_merge
 		      ? _("Your local changes to the following files would be overwritten by checkout:\n%%s"
@@ -119,7 +122,7 @@ void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
 			  "Please commit your changes or stash them before you %s.")
 		      : _("Your local changes to the following files would be overwritten by %s:\n%%s");
 	msgs[ERROR_WOULD_OVERWRITE] = msgs[ERROR_NOT_UPTODATE_FILE] =
-		xstrfmt(msg, cmd, cmd);
+		argv_array_pushf(&opts->msgs_to_free, msg, cmd, cmd);
 
 	msgs[ERROR_NOT_UPTODATE_DIR] =
 		_("Updating the following directories would lose untracked files in them:\n%s");
@@ -139,7 +142,8 @@ void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
 		      ? _("The following untracked working tree files would be removed by %s:\n%%s"
 			  "Please move or remove them before you %s.")
 		      : _("The following untracked working tree files would be removed by %s:\n%%s");
-	msgs[ERROR_WOULD_LOSE_UNTRACKED_REMOVED] = xstrfmt(msg, cmd, cmd);
+	msgs[ERROR_WOULD_LOSE_UNTRACKED_REMOVED] =
+		argv_array_pushf(&opts->msgs_to_free, msg, cmd, cmd);
 
 	if (!strcmp(cmd, "checkout"))
 		msg = advice_commit_before_merge
@@ -156,7 +160,8 @@ void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
 		      ? _("The following untracked working tree files would be overwritten by %s:\n%%s"
 			  "Please move or remove them before you %s.")
 		      : _("The following untracked working tree files would be overwritten by %s:\n%%s");
-	msgs[ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN] = xstrfmt(msg, cmd, cmd);
+	msgs[ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN] =
+		argv_array_pushf(&opts->msgs_to_free, msg, cmd, cmd);
 
 	/*
 	 * Special case: ERROR_BIND_OVERLAP refers to a pair of paths, we
@@ -179,6 +184,12 @@ void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
 		opts->unpack_rejects[i].strdup_strings = 1;
 }
 
+void clear_unpack_trees_porcelain(struct unpack_trees_options *opts)
+{
+	argv_array_clear(&opts->msgs_to_free);
+	memset(opts->msgs, 0, sizeof(opts->msgs));
+}
+
 static int do_add_entry(struct unpack_trees_options *o, struct cache_entry *ce,
 			 unsigned int set, unsigned int clear)
 {
diff --git a/unpack-trees.h b/unpack-trees.h
index 41178ada94..c2b434c606 100644
--- a/unpack-trees.h
+++ b/unpack-trees.h
@@ -2,7 +2,7 @@
 #define UNPACK_TREES_H
 
 #include "tree-walk.h"
-#include "string-list.h"
+#include "argv-array.h"
 
 #define MAX_UNPACK_TREES 8
 
@@ -33,6 +33,11 @@ enum unpack_trees_error_types {
 void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
 				  const char *cmd);
 
+/*
+ * Frees resources allocated by setup_unpack_trees_porcelain().
+ */
+void clear_unpack_trees_porcelain(struct unpack_trees_options *opts);
+
 struct unpack_trees_options {
 	unsigned int reset,
 		     merge,
@@ -57,6 +62,7 @@ struct unpack_trees_options {
 	struct pathspec *pathspec;
 	merge_fn_t fn;
 	const char *msgs[NB_UNPACK_TREES_ERROR_TYPES];
+	struct argv_array msgs_to_free;
 	/*
 	 * Store error messages in an array, each case
 	 * corresponding to a error message type