diff --git a/src/inflate.c b/src/inflate.c
index 870f89b..4fd3f3c 100644
--- a/src/inflate.c
+++ b/src/inflate.c
@@ -1504,9 +1504,10 @@
 {
     struct inflate_state FAR *state;
 
-    if (strm == Z_NULL || strm->state == Z_NULL) return -1L << 16;
+    if (strm == Z_NULL || strm->state == Z_NULL)
+        return (long)(((unsigned long)0 - 1) << 16);
     state = (struct inflate_state FAR *)strm->state;
-    return ((long)(state->back) << 16) +
+    return (long)(((unsigned long)((long)state->back)) << 16) +
         (state->mode == COPY ? state->length :
             (state->mode == MATCH ? state->was - state->length : 0));
 }