diff --git a/debian/changelog b/debian/changelog
index df929fd..a1bf2ff 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -16,6 +16,8 @@ flexnbd (0.2.0) UNRELEASED; urgency=medium
     disconnecting and retrying (#36, !40)
   * Ignore ends of discs that stray outside of 512-byte sector sizes (!42). 
   * Tweak logging for readloop failures (!44)
+  * Alter semantics of NBD_MAX_BLOCK_SIZE to remove struct size overheads when
+    calculating if a request exceeds the max block size (!45)
 
  -- James Carter <james.carter@bytemark.co.uk>  Thu, 11 Jan 2018 10:05:35 +0000
 
diff --git a/src/proxy/proxy.c b/src/proxy/proxy.c
index b323c68..291acd6 100644
--- a/src/proxy/proxy.c
+++ b/src/proxy/proxy.c
@@ -76,8 +76,12 @@ struct proxier* proxy_create(
 	}
 
 	out->init.buf = xmalloc( sizeof( struct nbd_init_raw ) );
-	out->req.buf  = xmalloc( NBD_MAX_SIZE );
-	out->rsp.buf  = xmalloc( NBD_MAX_SIZE );
+
+	/* Add on the request / reply size to our malloc to accommodate both
+	 * the struct and the data
+	 */
+	out->req.buf  = xmalloc( NBD_MAX_SIZE + NBD_REQUEST_SIZE );
+	out->rsp.buf  = xmalloc( NBD_MAX_SIZE + NBD_REPLY_SIZE );
 
 	log_context = xmalloc( strlen(s_upstream_address) + strlen(s_upstream_port) + 2 );
 	sprintf(log_context, "%s:%s", s_upstream_address, s_upstream_port);
@@ -452,15 +456,18 @@ int proxy_read_from_downstream( struct proxier *proxy, int state )
 			return EXIT;
 		}
 
-		/* Simple validations */
+		/* Simple validations -- the request / reply size have already
+		 * been taken into account in the xmalloc, so no need to worry
+		 * about them here
+		 */
 		if ( request->type == REQUEST_READ ) {
-			if (request->len > ( NBD_MAX_SIZE - NBD_REPLY_SIZE ) ) {
+			if ( request->len > NBD_MAX_SIZE ) {
 				warn( "NBD read request size %"PRIu32" too large", request->len );
 				return EXIT;
 			}
 		}
 		if ( request->type == REQUEST_WRITE ) {
-			if (request->len > ( NBD_MAX_SIZE - NBD_REQUEST_SIZE ) ) {
+			if ( request->len > NBD_MAX_SIZE ) {
 				warn( "NBD write request size %"PRIu32" too large", request->len );
 				return EXIT;
 			}
diff --git a/tests/acceptance/proxy_tests.rb b/tests/acceptance/proxy_tests.rb
index 65fe38a..897c487 100644
--- a/tests/acceptance/proxy_tests.rb
+++ b/tests/acceptance/proxy_tests.rb
@@ -207,4 +207,17 @@ module ProxyTests
       end
     end
   end
+
+  def test_maximum_write_request_size
+    # Defined in src/common/nbdtypes.h
+    nbd_max_block_size = 32 * 1024 * 1024
+    @env.writefile1('0' * 40 * 1024)
+    with_proxied_client do |client|
+      # This will crash with EPIPE if the proxy dies.
+      client.write(0, b * nbd_max_block_size)
+      rsp = client.read_response
+      assert_equal FlexNBD::REPLY_MAGIC, rsp[:magic]
+      assert_equal 0, rsp[:error]
+    end
+  end
 end