diff --git a/os/ports/RVCT/ARMCMx/chcoreasm_v6m.s b/os/ports/RVCT/ARMCMx/chcoreasm_v6m.s
index 725189996..a20327c5d 100644
--- a/os/ports/RVCT/ARMCMx/chcoreasm_v6m.s
+++ b/os/ports/RVCT/ARMCMx/chcoreasm_v6m.s
@@ -113,7 +113,7 @@ _port_switch_from_isr PROC
                 ldr     r2, =SCB_ICSR
                 movs    r3, #128
 #if CORTEX_ALTERNATE_SWITCH
-                lsls    r3, r3, #20
+                lsls    r3, r3, #21
                 str     r3, [r2, #0]
                 cpsie   i
 #else