diff --git a/libs/ardour/rt_midibuffer.cc b/libs/ardour/rt_midibuffer.cc
index 38222dc8fe..d1938e307e 100644
--- a/libs/ardour/rt_midibuffer.cc
+++ b/libs/ardour/rt_midibuffer.cc
@@ -383,7 +383,7 @@ RTMidiBuffer::alloc_blob (uint32_t size)
 	}
 
 	uint32_t offset = _pool_size;
-#if defined(__arm__) || defined(__aarch64_)
+#if defined(__arm__) || defined(__aarch64__)
 		_pool_size += ((size - 1) | 3) + 1;
 #else
 		_pool_size += size;