diff --git a/src/lzma/help.c b/src/lzma/help.c
index ad7dd861..f2a30ff6 100644
--- a/src/lzma/help.c
+++ b/src/lzma/help.c
@@ -140,13 +140,14 @@ These aren't implemented yet.
 	if (mem_limit == 0)
 		mem_limit = 1;
 
+	// We use PRIu64 instead of %zu to support pre-C99 libc.
 	puts(_("On this system and configuration, the tool will use"));
-	printf(_("  * roughly %zu MiB of memory at maximum; and\n"),
-			mem_limit);
-	printf(N_(
-		"  * at maximum of one thread for (de)compression.\n\n",
-		"  * at maximum of %zu threads for (de)compression.\n\n",
-		opt_threads), opt_threads);
+	printf(_("  * roughly %" PRIu64 " MiB of memory at maximum; and\n"),
+			(uint64_t)(mem_limit));
+	printf(N_("  * at maximum of one thread for (de)compression.\n\n",
+		"  * at maximum of %" PRIu64
+		" threads for (de)compression.\n\n",
+		(uint64_t)(opt_threads)), (uint64_t)(opt_threads));
 
 	printf(_("Report bugs to <%s> (in English or Finnish).\n"),
 			PACKAGE_BUGREPORT);
diff --git a/src/lzmadec/lzmadec.c b/src/lzmadec/lzmadec.c
index 93eed090..fef49249 100644
--- a/src/lzmadec/lzmadec.c
+++ b/src/lzmadec/lzmadec.c
@@ -108,10 +108,12 @@ help(void)
 "\n"
 "With no FILE, or when FILE is -, read standard input.\n"
 "\n"
-"On this configuration, the tool will use about %zu MiB of memory at maximum.\n"
+"On this configuration, the tool will use about %" PRIu64
+		" MiB of memory at maximum.\n"
 "\n"
 "Report bugs to <" PACKAGE_BUGREPORT "> (in English or Finnish).\n",
-		argv0, (mem_limit + 512 * 1024) / (1024 * 1024));
+		argv0, (uint64_t)((mem_limit + 512 * 1024) / (1024 * 1024)));
+		// Using PRIu64 above instead of %zu to support pre-C99 libc.
 	exit(0);
 }