diff --git a/CHANGES b/CHANGES
index 23211a9d7..641bd96a7 100644
--- a/CHANGES
+++ b/CHANGES
@@ -20,7 +20,7 @@ Bugfixes:
  - FFmpeg: Fix overflow and general issues with audio encoding
  - Qt: Fix crash when changing audio settings after a game is closed
  - GBA BIOS: Fix ArcTan sign in HLE BIOS
- - GBA BIOS: Fix ArcTan2 sign in HLE BIOS
+ - GBA BIOS: Fix ArcTan2 sign in HLE BIOS (fixes mgba.io/i/689)
 Misc:
  - Qt: Improved HiDPI support
  - Feature: Support ImageMagick 7
diff --git a/src/gba/bios.c b/src/gba/bios.c
index b2797827c..77a89129c 100644
--- a/src/gba/bios.c
+++ b/src/gba/bios.c
@@ -358,7 +358,8 @@ void GBASwi16(struct ARMCore* cpu, int immediate) {
 		cpu->gprs[0] = _ArcTan(cpu->gprs[0]);
 		break;
 	case 0xA:
-		cpu->gprs[0] = _ArcTan2(cpu->gprs[0], cpu->gprs[1]);
+		cpu->gprs[0] = (uint16_t) _ArcTan2(cpu->gprs[0], cpu->gprs[1]);
+		cpu->gprs[3] = 0x170;
 		break;
 	case 0xB:
 	case 0xC: