diff --git a/src/lpc11xx.c b/src/lpc11xx.c
index 9adbe2b..f7d72fa 100644
--- a/src/lpc11xx.c
+++ b/src/lpc11xx.c
@@ -216,7 +216,7 @@ lpc11xx_flash_write(struct target_s *target, uint32_t dest, const uint8_t *src,
 			chunk_offset = 0;
 
 			/* if we are programming the vectors, calculate the magic number */
-			if (dest == 0) {
+			if (chunk * IAP_PGM_CHUNKSIZE == 0) {
 				uint32_t *w = (uint32_t *)(&flash_pgm.data[0]);
 				uint32_t sum = 0;