diff --git a/misc.h b/misc.h
index 894344a2..63ed4e29 100644
--- a/misc.h
+++ b/misc.h
@@ -66,8 +66,10 @@
 #  define _tzcnt_u32(x) __tzcnt_u32(x)
 #  define _tzcnt_u64(x) __tzcnt_u64(x)
 #  define  _blsr_u32(x)  __blsr_u32(x)
+#ifndef _blsr_u64
 #  define  _blsr_u64(x)  __blsr_u64(x)
 # endif
+# endif
 #endif
 
 #endif // CRYPTOPP_DOXYGEN_PROCESSING