diff --git a/pattern.c b/pattern.c
index b0ea039..9904b80 100644
--- a/pattern.c
+++ b/pattern.c
@@ -913,6 +913,8 @@ static const char *validate_desc(const game_params *params, const char *desc)
                 p = desc;
                 while (*desc && isdigit((unsigned char)*desc)) desc++;
                 n = atoi(p);
+                if (n < 0)
+                    return "at least one clue is negative";
                 if (n > INT_MAX - 1)
                     return "at least one clue is grossly excessive";
                 rowspace -= n+1;