diff --git a/src/block.h b/src/block.h
index f5526d6a..dea57ff7 100644
--- a/src/block.h
+++ b/src/block.h
@@ -23,6 +23,14 @@
 /** Maximum ciphertext size without the length of the size field */
 #define MAX_BLOCK_SIZE_FIELD_VALUE (MAX_CIPHERTEXT_BLOCK_SIZE - sizeof(uint16_t))
 
+/*
+ * CONFIG_POUCH_BLOCK_SIZE feeds LOG2() above, which rounds DOWN to a power of two.
+ * A non-power-of-two value silently yields a smaller block everywhere it is used. Reject it at build
+ * time so the configured size is the size actually used.
+ */
+POUCH_STATIC_ASSERT((CONFIG_POUCH_BLOCK_SIZE & (CONFIG_POUCH_BLOCK_SIZE - 1)) == 0,
+                    "CONFIG_POUCH_BLOCK_SIZE must be a power of two");
+
 int block_decode_hdr(struct pouch_bufview *v,
                      uint16_t *block_size,
                      uint8_t *stream_id,
diff --git a/src/gateway/uplink.c b/src/gateway/uplink.c
index bc7177c3..06085882 100644
--- a/src/gateway/uplink.c
+++ b/src/gateway/uplink.c
@@ -16,6 +16,7 @@
 
 #include "uplink.h"
 #include "../buf.h"
+#include "../block.h"
 
 POUCH_LOG_REGISTER(gw_uplink, CONFIG_POUCH_GATEWAY_LOG_LEVEL);
 
@@ -191,11 +192,11 @@ static void send_uplink_via_cloud(struct pouch_gateway_uplink *uplink)
 }
 
 /*
- * Max bytes per gateway block.  Each pouch_buf slot holds at least
- * CONFIG_POUCH_BLOCK_SIZE bytes; we conservatively use that as the
+ * Max payload bytes per gateway block.  Each pouch_buf slot holds at least
+ * MAX_BLOCK_PAYLOAD_SIZE bytes; we conservatively use that as the
  * per-block capacity.
  */
-#define GW_BLOCK_MAX_BYTES CONFIG_POUCH_BLOCK_SIZE
+#define GW_BLOCK_MAX_BYTES MAX_BLOCK_PAYLOAD_SIZE
 
 int pouch_gateway_uplink_write(struct pouch_gateway_uplink *uplink,
                                const uint8_t *payload,