diff --git a/cpu/msp430/Makefile.msp430 b/cpu/msp430/Makefile.msp430
index 1dc82798b..fbe5a1969 100644
--- a/cpu/msp430/Makefile.msp430
+++ b/cpu/msp430/Makefile.msp430
@@ -81,10 +81,10 @@ endif
 ifndef IAR_DATA16_HEAP_SIZE
  IAR_DATA16_HEAP_SIZE=100
 endif
-LDFLAGSNO += -D_STACK_SIZE=$(IAR_STACK_SIZE) -D_DATA16_HEAP_SIZE=$(IAR_DATA16_HEAP_SIZE)
-ifdef IAR_DATA20_HEAP_SIZE
- LDFLAGSNO += -D_DATA20_HEAP_SIZE=$(IAR_DATA20_HEAP_SIZE)
+ifndef IAR_DATA20_HEAP_SIZE
+ IAR_DATA20_HEAP_SIZE=0
 endif
+LDFLAGSNO += -D_STACK_SIZE=$(IAR_STACK_SIZE) -D_DATA16_HEAP_SIZE=$(IAR_DATA16_HEAP_SIZE) -D_DATA20_HEAP_SIZE=$(IAR_DATA20_HEAP_SIZE)
 
 CUSTOM_RULE_C_TO_O = 1
 %.o: %.c