diff --git a/sys/bitfield/bitfield.c b/sys/bitfield/bitfield.c
index bb33165805..c685e28d8d 100644
--- a/sys/bitfield/bitfield.c
+++ b/sys/bitfield/bitfield.c
@@ -90,7 +90,18 @@ void bf_set_all(uint8_t field[], size_t size)
 
     memset(field, 0xff, bytes);
     if (bits) {
-        field[bytes] = ~((1U << (8 - bits)) - 1);
+        field[bytes] |= ~((1U << (8 - bits)) - 1);
+    }
+}
+
+void bf_clear_all(uint8_t field[], size_t size)
+{
+    unsigned bytes = size >> 3;
+    unsigned bits = size & 0x7;
+
+    memset(field, 0, bytes);
+    if (bits) {
+        field[bytes] &= ((1U << (8 - bits)) - 1);
     }
 }
 
diff --git a/sys/include/bitfield.h b/sys/include/bitfield.h
index 087597d5ab..19c952eb8c 100644
--- a/sys/include/bitfield.h
+++ b/sys/include/bitfield.h
@@ -365,10 +365,7 @@ static inline void bf_set_all_atomic(uint8_t field[], size_t size)
  * @param[in]     field The bitfield
  * @param[in]     size  The number of bits in the bitfield
  */
-static inline void bf_clear_all(uint8_t field[], size_t size)
-{
-    memset(field, 0, (size + 7) / 8);
-}
+void bf_clear_all(uint8_t field[], size_t size);
 
 /**
  * @brief  Atomically clear all bits in the bitfield to 0