diff --git a/core/hwtimer.c b/core/hwtimer.c
index c68f2fa325..3c7152b458 100644
--- a/core/hwtimer.c
+++ b/core/hwtimer.c
@@ -150,16 +150,12 @@ static int _hwtimer_set(unsigned long offset, void (*callback)(void*), void *ptr
 
     unsigned state;
 
-    if (!inISR()) {
-        state = disableIRQ();
-    }
+    state = disableIRQ();
 
     int n = lifo_get(lifo);
 
     if (n == -1) {
-        if (!inISR()) {
-            restoreIRQ(state);
-        }
+        restoreIRQ(state);
 
         puts("No hwtimer left.");
         return -1;
@@ -179,9 +175,7 @@ static int _hwtimer_set(unsigned long offset, void (*callback)(void*), void *ptr
 
     lpm_prevent_sleep++;
 
-    if (!inISR()) {
-        restoreIRQ(state);
-    }
+    restoreIRQ(state);
 
     return n;
 }
diff --git a/core/include/arch/irq_arch.h b/core/include/arch/irq_arch.h
index 554fb2ce59..d57db05405 100644
--- a/core/include/arch/irq_arch.h
+++ b/core/include/arch/irq_arch.h
@@ -16,6 +16,9 @@
  * This file acts as a wrapper between the kernels interrupt interface and the architecture
  * dependent implementation of the interfaces.
  *
+ * @note All functions in this module have to be implemented in a way that it
+ *       is safe to call them from within the context of an ISR.
+ *
  * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
  */