diff --git a/flood.c b/flood.c
index 67181a8..1b0fa6e 100644
--- a/flood.c
+++ b/flood.c
@@ -210,6 +210,8 @@ static const char *validate_params(const game_params *params, bool full)
 {
     if (params->w < 2 && params->h < 2)
         return "Grid must contain at least two squares";
+    if (params->w < 1 || params->h < 1)
+        return "Width and height must be at least one";
     if (params->w > INT_MAX / params->h)
         return "Width times height must not be unreasonably large";
     if (params->colours < 3 || params->colours > 10)