diff --git a/src/f6502.c b/src/f6502.c
index 74a51d1..2a422b5 100644
--- a/src/f6502.c
+++ b/src/f6502.c
@@ -434,7 +434,7 @@ static inline int f6502_interrupt(f6502_Core* core,
                                   uint16_t addr) {
     PUSH16(core, core->registers.S, core->registers.PC);
     PUSH(core, core->registers.S,
-         core->registers.P & ~f6502_Status_B);
+         (core->registers.P & ~f6502_Status_B) | f6502_Status_1);
     SET(core->registers.P, f6502_Status_I);
     core->registers.PC = f6502_read16(&core->memory, addr);
     return 7;