diff --git a/pycardium/mphalport.c b/pycardium/mphalport.c
index a45a8776365477251ee801ef8ca06ecde131293e..f8370ff11de1bbb0ff62c0631aa058ac81d48965 100644
--- a/pycardium/mphalport.c
+++ b/pycardium/mphalport.c
@@ -169,7 +169,7 @@ static int64_t systick_get_us()
 	 * its value can jump to 0 before the interrupt is triggered.
 	 * Simply wait until it is not 0 and then read the count. */
 	do {
-		__set_PRIMASK(0);
+		__set_PRIMASK(1);
 		val   = SysTick->VAL;
 		count = systick_count;
 		__set_PRIMASK(irqsaved);