diff --git a/stmhal/pendsv.c b/stmhal/pendsv.c
index 4c2a14de1b..200d13a5a9 100644
--- a/stmhal/pendsv.c
+++ b/stmhal/pendsv.c
@@ -106,11 +106,14 @@ void pendsv_isr_handler(void) {
         ".no_obj:\n"                    // pendsv_object==NULL
         "push {r4-r11, lr}\n"
         "vpush {s16-s31}\n"
+        "mrs r5, primask\n"             // save PRIMASK in r5
+        "cpsid i\n"                     // disable interrupts while we change stacks
         "mov r0, sp\n"                  // pass sp to save
         "mov r4, lr\n"                  // save lr because we are making a call
         "bl pyb_thread_next\n"          // get next thread to execute
         "mov lr, r4\n"                  // restore lr
         "mov sp, r0\n"                  // switch stacks
+        "msr primask, r5\n"             // reenable interrupts
         "vpop {s16-s31}\n"
         "pop {r4-r11, lr}\n"
         "bx lr\n"                       // return from interrupt; will return to new thread