diff --git a/include/mbedtls/bn_mul.h b/include/mbedtls/bn_mul.h
index e04926043..438aa8cea 100644
--- a/include/mbedtls/bn_mul.h
+++ b/include/mbedtls/bn_mul.h
@@ -49,7 +49,14 @@
 /* armcc5 --gnu defines __GNUC__ but doesn't support GNU's extended asm */
 #if defined(__GNUC__) && \
     ( !defined(__ARMCC_VERSION) || __ARMCC_VERSION >= 6000000 )
-#if defined(__i386__)
+
+/*
+ * Disable use of the i386 assembly code below if option -O0, to disable all
+ * compiler optimisations, is passed, detected with __OPTIMIZE__
+ * This is done as the number of registers used in the assembly code doesn't
+ * work with the -O0 option.
+ */
+#if defined(__i386__) && !defined(__OPTIMIZE__)
 
 #define MULADDC_INIT                        \
     asm(                                    \