diff --git a/sys/include/phydat.h b/sys/include/phydat.h
index b83f11c2cc..db8036a06c 100644
--- a/sys/include/phydat.h
+++ b/sys/include/phydat.h
@@ -277,11 +277,11 @@ void phydat_fit(phydat_t *dat, const int32_t *values, unsigned int dim);
  * for @ref UNIT_NONE.
  *
  * @param[in]  data     data to encode
- * @param[in]  dim      dimensions used in @p data, MUST be > 0 and < PHYDAT_DIM
+ * @param[in]  dim      dimensions used in @p data, MUST be > 0 and <= PHYDAT_DIM
  * @param[out] buf      target buffer for the JSON string, or NULL
  *
  * @pre     @p dim > 0
- * @pre     @p dim < PHYDAT_DIM
+ * @pre     @p dim <= PHYDAT_DIM
  *
  * @return  number of bytes (potentially) written to @p buf, including `\0`
  *          terminator
diff --git a/sys/phydat/phydat_json.c b/sys/phydat/phydat_json.c
index d2c3675772..176016f8c0 100644
--- a/sys/phydat/phydat_json.c
+++ b/sys/phydat/phydat_json.c
@@ -43,7 +43,7 @@ static size_t _bool_to_str(int16_t val, char *buf)
 
 size_t phydat_to_json(const phydat_t *data, size_t dim, char *buf)
 {
-    assert((dim > 0) && (dim < PHYDAT_DIM));
+    assert((dim > 0) && (dim <= PHYDAT_DIM));
 
     size_t pos = 0;