diff --git a/3rdparty/p256-m/p256-m/p256-m.c b/3rdparty/p256-m/p256-m/p256-m.c
index 53d306f63..21a021bad 100644
--- a/3rdparty/p256-m/p256-m/p256-m.c
+++ b/3rdparty/p256-m/p256-m/p256-m.c
@@ -199,10 +199,12 @@ static uint64_t u32_muladd64(uint32_t x, uint32_t y, uint32_t z, uint32_t t);
  * Currently assembly optimisations are only supported with GCC/Clang for
  * Arm's Cortex-A and Cortex-M lines of CPUs, which start with the v6-M and
  * v7-M architectures. __ARM_ARCH_PROFILE is not defined for v6 and earlier.
+ * Thumb and 32-bit assembly is supported; aarch64 is not supported.
  */
 #if defined(__GNUC__) &&\
     defined(__ARM_ARCH) && __ARM_ARCH >= 6 && defined(__ARM_ARCH_PROFILE) && \
-    ( __ARM_ARCH_PROFILE == 77 || __ARM_ARCH_PROFILE == 65 ) /* 'M' or 'A' */
+    ( __ARM_ARCH_PROFILE == 77 || __ARM_ARCH_PROFILE == 65 ) /* 'M' or 'A' */ && \
+    !defined(__aarch64__)
 
 /*
  * This set of CPUs is conveniently partitioned as follows: