diff --git a/crypto/internal.h b/crypto/internal.h
index c4e2e517..3f0ff6d1 100644
--- a/crypto/internal.h
+++ b/crypto/internal.h
@@ -151,6 +151,19 @@ extern "C" {
 #endif
 
 
+#if defined(OPENSSL_MSAN) && !defined(OPENSSL_NO_ASM)
+// MSan works by instrumenting memory accesses in the compiler. Accesses from
+// uninstrumented code, such as assembly, are invisible to it. MSan will
+// incorrectly report reads from assembly-initialized memory as uninitialized.
+// If building BoringSSL with MSan, exclude assembly files from the build and
+// define OPENSSL_NO_ASM.
+//
+// This is checked here rather than in base.h because the consumer might not
+// define OPENSSL_NO_ASM. It is only necessary for BoringSSL source files to be
+// built with it.
+#error "BoringSSL must be built with assembly disabled to use MSan."
+#endif
+
 #if defined(OPENSSL_X86) || defined(OPENSSL_X86_64) || defined(OPENSSL_ARM) || \
     defined(OPENSSL_AARCH64) || defined(OPENSSL_PPC64LE)
 // OPENSSL_cpuid_setup initializes the platform-specific feature cache.