diff --git a/deflate.c b/deflate.c
index 06ca0ee..b126a71 100644
--- a/deflate.c
+++ b/deflate.c
@@ -1322,6 +1322,8 @@ local void fill_window(s)
     unsigned more;    /* Amount of free space at the end of the window. */
     uInt wsize = s->w_size;
 
+    Assert(s->lookahead < MIN_LOOKAHEAD, "already enough lookahead");
+
     do {
         more = (unsigned)(s->window_size -(ulg)s->lookahead -(ulg)s->strstart);
 
@@ -1439,6 +1441,9 @@ local void fill_window(s)
             s->high_water += init;
         }
     }
+
+    Assert((ulg)s->strstart <= s->window_size - MIN_LOOKAHEAD,
+           "not enough room for search");
 }
 
 /* ===========================================================================