diff --git a/makefiles/toolchain_gnu-arm-eabi.mk b/makefiles/toolchain_gnu-arm-eabi.mk
index b38f40eb4c..7e1f510e6d 100644
--- a/makefiles/toolchain_gnu-arm-eabi.mk
+++ b/makefiles/toolchain_gnu-arm-eabi.mk
@@ -124,7 +124,7 @@ ARCHOPTIMIZATION	 = $(MAXOPTIMIZATION) \
 
 # enable precise stack overflow tracking
 # note - requires corresponding support in NuttX
-INSTRUMENTATIONDEFINES	 ?= $(ARCHINSTRUMENTATIONDEFINES_$(CONFIG_ARCH))
+INSTRUMENTATIONDEFINES	 = $(ARCHINSTRUMENTATIONDEFINES_$(CONFIG_ARCH))
 
 # Language-specific flags
 #