diff --git a/src/mame/machine/segaic16.c b/src/mame/machine/segaic16.c
index 2b061ea41ff..1ab0e8a5c0f 100644
--- a/src/mame/machine/segaic16.c
+++ b/src/mame/machine/segaic16.c
@@ -295,11 +295,12 @@ static UINT16 memory_mapper_r(struct memory_mapper_chip *chip, offs_t offset, UI
 static void update_memory_mapping(running_machine *machine, struct memory_mapper_chip *chip, int decrypt)
 {
 	int rgnum;
+	address_space *space = cpu_get_address_space(chip->cpu, ADDRESS_SPACE_PROGRAM);
 
 	if (LOG_MEMORY_MAP) mame_printf_debug("----\nRemapping:\n");
 
 	/* first reset everything back to the beginning */
-	memory_install_readwrite16_handler(cpu_get_address_space(chip->cpu, ADDRESS_SPACE_PROGRAM), 0x000000, 0xffffff, 0, 0, segaic16_memory_mapper_lsb_r, segaic16_memory_mapper_lsb_w);
+	memory_install_readwrite16_handler(space, 0x000000, 0xffffff, 0, 0, segaic16_memory_mapper_lsb_r, segaic16_memory_mapper_lsb_w);
 
 	/* loop over the regions */
 	for (rgnum = 0; chip->map[rgnum].regbase != 0; rgnum++)
@@ -328,13 +329,18 @@ static void update_memory_mapping(running_machine *machine, struct memory_mapper
 
 		/* map it */
 		if (read != NULL)
-			memory_install_read16_handler(cpu_get_address_space(chip->cpu, ADDRESS_SPACE_PROGRAM), region_start, region_end, 0, region_mirror, read);
+			memory_install_read16_handler(space, region_start, region_end, 0, region_mirror, read);
 		else if (readbank != NULL)
-			memory_install_read_bank(cpu_get_address_space(chip->cpu, ADDRESS_SPACE_PROGRAM), region_start, region_end, 0, region_mirror, readbank);
+			memory_install_read_bank(space, region_start, region_end, 0, region_mirror, readbank);
+		else
+			memory_install_read16_handler(space, region_start, region_end, 0, region_mirror, segaic16_open_bus_r);
+
 		if (write != NULL)
-			memory_install_write16_handler(cpu_get_address_space(chip->cpu, ADDRESS_SPACE_PROGRAM), region_start, region_end, 0, region_mirror, write);
+			memory_install_write16_handler(space, region_start, region_end, 0, region_mirror, write);
 		else if (writebank != NULL)
-			memory_install_write_bank(cpu_get_address_space(chip->cpu, ADDRESS_SPACE_PROGRAM), region_start, region_end, 0, region_mirror, writebank);
+			memory_install_write_bank(space, region_start, region_end, 0, region_mirror, writebank);
+		else
+			memory_unmap_write(space, region_start, region_end, 0, region_mirror);
 
 		/* set the bank pointer */
 		if (readbank != NULL)