diff --git a/boards/Makefile.include.cortexm_common b/boards/Makefile.include.cortexm_common
index 9273b7e198d2bd47bcb1c18e3f1a45d20cc108aa..05c5ca78685faa61a36a51dac80ede57e5fa04a7 100644
--- a/boards/Makefile.include.cortexm_common
+++ b/boards/Makefile.include.cortexm_common
@@ -9,7 +9,7 @@ include $(RIOTBOARD)/Makefile.include.gnu
 
 # define build specific options
 export CFLAGS_CPU   = -mcpu=$(MCPU) -mlittle-endian -mthumb -mno-thumb-interwork $(CFLAGS_FPU)
-export CFLAGS_STYLE = -std=gnu99 -Wall -Wstrict-prototypes
+export CFLAGS_STYLE = -std=gnu99 -Wall -Wstrict-prototypes -Werror=implicit-function-declaration
 export CFLAGS_LINK  = -ffunction-sections -fdata-sections -fno-builtin
 export CFLAGS_DBG   = -ggdb -g3
 export CFLAGS_OPT   ?= -Os