diff --git a/dummyflasher.c b/dummyflasher.c
index 2f39c9888..4661f3d3e 100644
--- a/dummyflasher.c
+++ b/dummyflasher.c
@@ -668,6 +668,7 @@ int dummy_init(void)
 #if EMULATE_CHIP
 	struct stat image_stat;
 #endif
+	char *endptr;
 
 	struct emu_data *data = calloc(1, sizeof(struct emu_data));
 	if (!data) {
@@ -710,9 +711,9 @@ int dummy_init(void)
 
 	tmp = extract_programmer_param("spi_write_256_chunksize");
 	if (tmp) {
-		data->spi_write_256_chunksize = atoi(tmp);
+		data->spi_write_256_chunksize = strtoul(tmp, &endptr, 0);
 		free(tmp);
-		if (data->spi_write_256_chunksize < 1) {
+		if (*endptr != '\0' || data->spi_write_256_chunksize < 1) {
 			msg_perr("invalid spi_write_256_chunksize\n");
 			return 1;
 		}
@@ -977,7 +978,6 @@ int dummy_init(void)
 #ifdef EMULATE_SPI_CHIP
 	status = extract_programmer_param("spi_status");
 	if (status) {
-		char *endptr;
 		errno = 0;
 		data->emu_status = strtoul(status, &endptr, 0);
 		free(status);