diff --git a/signpost.c b/signpost.c
index 1c8cfee..daf5509 100644
--- a/signpost.c
+++ b/signpost.c
@@ -7,6 +7,7 @@
 #include <string.h>
 #include <assert.h>
 #include <ctype.h>
+#include <limits.h>
 #include <math.h>
 
 #include "puzzles.h"
@@ -421,6 +422,8 @@ static const char *validate_params(const game_params *params, bool full)
 {
     if (params->w < 1) return "Width must be at least one";
     if (params->h < 1) return "Height must be at least one";
+    if (params->w > INT_MAX / params->h)
+        return "Width times height must not be unreasonably large";
     if (full && params->w == 1 && params->h == 1)
 	/* The UI doesn't let us move these from unsolved to solved,
 	 * so we disallow generating (but not playing) them. */