diff --git a/crypto/math/datatypes.c b/crypto/math/datatypes.c
index 001584c..4fcb396 100644
--- a/crypto/math/datatypes.c
+++ b/crypto/math/datatypes.c
@@ -79,7 +79,7 @@ int octet_get_weight(uint8_t octet)
 
 /* the value MAX_PRINT_STRING_LEN is defined in datatypes.h */
 
-char bit_string[MAX_PRINT_STRING_LEN];
+static char bit_string[MAX_PRINT_STRING_LEN];
 
 uint8_t srtp_nibble_to_hex_char(uint8_t nibble)
 {
diff --git a/test/util.c b/test/util.c
index 2abc28e..c0f7614 100644
--- a/test/util.c
+++ b/test/util.c
@@ -49,7 +49,7 @@
 #include <stdint.h>
 
 /* include space for null terminator */
-char bit_string[MAX_PRINT_STRING_LEN + 1];
+static char bit_string[MAX_PRINT_STRING_LEN + 1];
 
 static inline int hex_char_to_nibble(uint8_t c)
 {