diff --git a/repl/makefile b/repl/makefile
index 1738a97..b8f0446 100644
--- a/repl/makefile
+++ b/repl/makefile
@@ -1,6 +1,6 @@
 #compiler settings
 CC=gcc
-CFLAGS+=-g -Wall -Werror -Wno-unused-parameter -Wno-unused-function -Wno-unused-variable
+CFLAGS+=-g -Wall -Werror -Wno-unused-parameter -Wno-unused-function -Wno-unused-variable -Wformat
 LIBS+=-lm -lToy
 LDFLAGS+=-Wl,-rpath,'$$ORIGIN'
 
diff --git a/source/makefile b/source/makefile
index 949fdf3..0006dfb 100644
--- a/source/makefile
+++ b/source/makefile
@@ -1,6 +1,6 @@
 #compiler settings
 CC=gcc
-CFLAGS+=-g -Wall -Werror -Wno-unused-parameter -Wno-unused-function -Wno-unused-variable
+CFLAGS+=-g -Wall -Werror -Wno-unused-parameter -Wno-unused-function -Wno-unused-variable -Wformat
 LIBS+=-lm
 LDFLAGS+=
 
diff --git a/tests/makefile b/tests/makefile
index d7e4262..b38868c 100644
--- a/tests/makefile
+++ b/tests/makefile
@@ -1,6 +1,6 @@
 #compiler settings
 CC=gcc
-CFLAGS=-g -Wall -Werror -Wno-unused-parameter -Wno-unused-function -Wno-unused-variable
+CFLAGS=-g -Wall -Werror -Wno-unused-parameter -Wno-unused-function -Wno-unused-variable -Wformat
 LIBS=-lm
 
 ifeq ($(shell uname),Linux)