diff --git a/crnlib/crn_packed_uint.h b/crnlib/crn_packed_uint.h
index dedca4ae..71146342 100644
--- a/crnlib/crn_packed_uint.h
+++ b/crnlib/crn_packed_uint.h
@@ -39,14 +39,15 @@ struct packed_uint {
   }
 
   inline operator unsigned int() const {
-    switch (sizeof(m_buf) / sizeof(m_buf[0])) {
+    CRNLIB_ASSUME(sizeof(m_buf) == N);
+    switch (N) {
       case 1:
         return m_buf[0];
       case 2:
         return (m_buf[0] << 8U) | m_buf[1];
       case 3:
         return (m_buf[0] << 16U) | (m_buf[1] << 8U) | (m_buf[2]);
-      case 4:
+      default:
         return (m_buf[0] << 24U) | (m_buf[1] << 16U) | (m_buf[2] << 8U) | (m_buf[3]);
     }
   }
diff --git a/inc/crn_defs.h b/inc/crn_defs.h
index 0a457ccb..699b64aa 100644
--- a/inc/crn_defs.h
+++ b/inc/crn_defs.h
@@ -225,14 +225,15 @@ struct crn_packed_uint {
   }
 
   inline operator unsigned int() const {
-    switch (sizeof(m_buf) / sizeof(m_buf[0])) {
+    static_assert(sizeof(m_buf) == N, "");
+    switch (N) {
       case 1:
         return m_buf[0];
       case 2:
         return (m_buf[0] << 8U) | m_buf[1];
       case 3:
         return (m_buf[0] << 16U) | (m_buf[1] << 8U) | (m_buf[2]);
-      case 4:
+      default:
         return (m_buf[0] << 24U) | (m_buf[1] << 16U) | (m_buf[2] << 8U) | (m_buf[3]);
     }
   }