diff --git a/usr.bin/fmt/fmt.c b/usr.bin/fmt/fmt.c
index 33f92b1b82d5..ed85c2e49f2a 100644
--- a/usr.bin/fmt/fmt.c
+++ b/usr.bin/fmt/fmt.c
@@ -176,6 +176,7 @@ static const char copyright[] =
 __FBSDID("$FreeBSD$");
 
 #include <err.h>
+#include <limits.h>
 #include <locale.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -351,6 +352,7 @@ main(int argc, char *argv[]) {
   }
   if (goal_length==0) goal_length = 65;
   if (max_length==0) max_length = goal_length+10;
+  if (max_length >= SIZE_T_MAX / sizeof (wchar_t)) errx(EX_USAGE, "max length too large");
   /* really needn't be longer */
   output_buffer = XMALLOC((max_length+1) * sizeof(wchar_t));