diff --git a/src/zrle.c b/src/zrle.c
index 9fd2be8..7faff31 100644
--- a/src/zrle.c
+++ b/src/zrle.c
@@ -178,8 +178,6 @@ void zrle_encode_tile(struct vec* dst, const struct rfb_pixel_format* dst_fmt,
 
 int zrle_deflate(struct vec* dst, const struct vec* src, z_stream* zs, bool flush)
 {
-	int r = Z_STREAM_ERROR;
-
 	zs->next_in = src->data;
 	zs->avail_in = src->len;
 
@@ -190,8 +188,9 @@ int zrle_deflate(struct vec* dst, const struct vec* src, z_stream* zs, bool flus
 		zs->next_out = ((Bytef*)dst->data) + dst->len;
 		zs->avail_out = dst->cap - dst->len;
 
-		r = deflate(zs, flush ? Z_SYNC_FLUSH : Z_NO_FLUSH);
-		assert(r != Z_STREAM_ERROR);
+		int r = deflate(zs, flush ? Z_SYNC_FLUSH : Z_NO_FLUSH);
+		if (r == Z_STREAM_ERROR)
+			return -1;
 
 		dst->len = zs->next_out - (Bytef*)dst->data;
 	} while (zs->avail_out == 0);