diff --git a/CMakeLists.txt b/CMakeLists.txt
index 88484de..b1b705d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,6 +41,7 @@ if (ENABLE_CUSTOM_COMPILER_FLAGS)
         -Wcomma
         -Wdouble-promotion
         -Wparentheses
+        -Wformat-overflow
         )
 endif()
 
diff --git a/tests/parse_hex4.c b/tests/parse_hex4.c
index 59b07ee..7115cba 100644
--- a/tests/parse_hex4.c
+++ b/tests/parse_hex4.c
@@ -31,8 +31,8 @@
 static void parse_hex4_should_parse_all_combinations(void)
 {
     unsigned int number = 0;
-    unsigned char digits_lower[5];
-    unsigned char digits_upper[5];
+    unsigned char digits_lower[6];
+    unsigned char digits_upper[6];
     /* test all combinations */
     for (number = 0; number <= 0xFFFF; number++)
     {