diff --git a/deflate.c b/deflate.c
index 7a9e05e..544521d 100644
--- a/deflate.c
+++ b/deflate.c
@@ -676,7 +676,7 @@ int ZEXPORT deflate (strm, flush)
     s = strm->state;
 
     if (strm->next_out == Z_NULL ||
-        (strm->next_in == Z_NULL && strm->avail_in != 0) ||
+        (strm->avail_in != 0 && strm->next_in == Z_NULL) ||
         (s->status == FINISH_STATE && flush != Z_FINISH)) {
         ERR_RETURN(strm, Z_STREAM_ERROR);
     }
diff --git a/gzwrite.c b/gzwrite.c
index ad164b2..efff93a 100644
--- a/gzwrite.c
+++ b/gzwrite.c
@@ -47,6 +47,7 @@ local int gz_init(state)
             gz_error(state, Z_MEM_ERROR, "out of memory");
             return -1;
         }
+        strm->next_in = NULL;
     }
 
     /* mark state as initialized */