diff --git a/src/common/util.c b/src/common/util.c
index 36468d2d84..ae385e1b94 100644
--- a/src/common/util.c
+++ b/src/common/util.c
@@ -3412,7 +3412,7 @@ format_helper_exit_status(unsigned char child_state, int saved_errno,
    * Count how many chars of space we have left, and keep a pointer into the
    * current point in the buffer.
    */
-  left = HEX_ERRNO_SIZE;
+  left = HEX_ERRNO_SIZE+1;
   cur = hex_errno;
 
   /* Emit child_state */
@@ -3712,7 +3712,7 @@ tor_spawn_background(const char *const filename, const char **argv,
      this is used for printing out the error message */
   unsigned char child_state = CHILD_STATE_INIT;
 
-  char hex_errno[HEX_ERRNO_SIZE + 1];
+  char hex_errno[HEX_ERRNO_SIZE + 2]; /* + 1 should be sufficient actually */
 
   static int max_fd = -1;
 
diff --git a/src/test/test_util.c b/src/test/test_util.c
index 6e1ee713d8..65d9d2f878 100644
--- a/src/test/test_util.c
+++ b/src/test/test_util.c
@@ -2252,6 +2252,7 @@ test_util_exit_status(void *ptr)
   n = format_helper_exit_status(0xFF, -0x80000000, hex_errno);
   test_streq("FF/-80000000\n", hex_errno);
   test_eq(n, strlen(hex_errno));
+  test_eq(n, HEX_ERRNO_SIZE);
 
   clear_hex_errno(hex_errno);
   n = format_helper_exit_status(0x7F, 0, hex_errno);