diff --git a/sys/clif/clif.c b/sys/clif/clif.c
index a52b5a9cac..f94a211aab 100644
--- a/sys/clif/clif.c
+++ b/sys/clif/clif.c
@@ -177,22 +177,17 @@ ssize_t clif_add_attr(clif_attr_t *attr, char *buf, size_t maxlen)
     assert(attr);
     assert(attr->key);
 
-    /* if no length given, calculate it */
-    if (!attr->key_len) {
-        attr->key_len = strlen(attr->key);
-    }
-
     /* count attr name size and separator ';' */
     size_t req_space = attr->key_len + 1;
     size_t pos = 0;
-    int quoted = strcmp(attr->key, LF_ATTR_SIZE) ? 1 : 0;
+    int quoted = 0;
+    if (attr->key_len >= LF_ATTR_SIZE_S) {
+        quoted = strcmp(attr->key, LF_ATTR_SIZE) ? 1 : 0;
+    }
 
     if (attr->value) {
-        if (!attr->value_len) {
-            attr->value_len = strlen(attr->value);
-        }
         /* count also '=' */
-        req_space += attr->value_len +  1;
+        req_space += attr->value_len + 1;
     }
 
     if (quoted) {
diff --git a/sys/include/clif.h b/sys/include/clif.h
index 12f772837c..95dab7b57d 100644
--- a/sys/include/clif.h
+++ b/sys/include/clif.h
@@ -237,9 +237,7 @@ ssize_t clif_add_target(const char *target, char *buf, size_t maxlen);
  * @note
  *       - If @p buf is NULL this will return the amount of bytes that would be
  *         needed.
- *       - If the lengths of the key or the value of the attribute are not
- *         defined a NULL-terminated string will be assumed, and it will be
- *         calculated.
+ *       - The length of the key must be set in `attr->key_len`.
  *
  * @return amount of bytes used from the buffer if successful
  * @return CLIF_NO_SPACE if there is not enough space in the buffer
diff --git a/tests/unittests/tests-clif/tests-clif.c b/tests/unittests/tests-clif/tests-clif.c
index 105ec5e592..861aa0c7cd 100644
--- a/tests/unittests/tests-clif/tests-clif.c
+++ b/tests/unittests/tests-clif/tests-clif.c
@@ -82,9 +82,9 @@ static void test_clif_encode_links(void)
     const char exp_string[] = "</sensor/temp>;rt=\"temperature\";if=\"sensor\","
                               "</node/info>,</node/ep>;ct=\"40\"";
     clif_attr_t attrs[] = {
-        { .key = "rt", .value = "temperature" },
-        { .key = "if", .value = "sensor" },
-        { .key = "ct", .value = "40" }
+        { .key = "rt", .key_len = 2, .value = "temperature", .value_len = 11 },
+        { .key = "if", .key_len = 2, .value = "sensor", .value_len = 6 },
+        { .key = "ct", .key_len = 2, .value = "40", .value_len = 2 }
     };
 
     clif_t links[] = {
@@ -317,12 +317,10 @@ static void test_clif_get_attr_empty(void)
 
 static void tests_clif_decode_encode_minimal(void)
 {
-    char input_buf[] = "</sensors>";
+    #define BUFF_SIZE 50
 
-    /* The required buffer size is (in this case) equal to the input buffer
-     * plus one additional byte to hold the null termination */
-    const size_t output_buf_size = strlen(input_buf) + 1;
-    char output_buf[output_buf_size];
+    char input_buf[] = "</sensors>";
+    char output_buf[BUFF_SIZE];
     clif_t out_link;
 
     ssize_t input_len = strlen(input_buf);
@@ -332,7 +330,7 @@ static void tests_clif_decode_encode_minimal(void)
         TEST_FAIL("Malformed input string");
     }
 
-    ssize_t result_len = clif_encode_link(&out_link, output_buf, output_buf_size);
+    ssize_t result_len = clif_encode_link(&out_link, output_buf, BUFF_SIZE);
     if (result_len == CLIF_NO_SPACE) {
         TEST_FAIL("No space left in the buffer");
     }