diff --git a/ChangeLog b/ChangeLog
index 305eef60b..eb9cdae2d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -29,6 +29,8 @@ Bugfix
      by Brendan Shanks. Part of a fix for #992.
    * Fix compilation error when MBEDTLS_ARC4_C is disabled and
      MBEDTLS_CIPHER_NULL_CIPHER is enabled. Found by TrinityTonic in #1719.
+   * Fix the inline assembly for the MPI multiply helper function for i386 and
+     i386 with SSE2. Found by László Langó. Fixes #1550
 
 Changes
    * Change the shebang line in Perl scripts to look up perl in the PATH.
diff --git a/include/mbedtls/bn_mul.h b/include/mbedtls/bn_mul.h
index f4b2b561d..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(                                    \
@@ -142,7 +149,7 @@
         "movl   %%esi, %3       \n\t"   \
         : "=m" (t), "=m" (c), "=m" (d), "=m" (s)        \
         : "m" (t), "m" (s), "m" (d), "m" (c), "m" (b)   \
-        : "eax", "ecx", "edx", "esi", "edi"             \
+        : "eax", "ebx", "ecx", "edx", "esi", "edi"      \
     );
 
 #else
@@ -154,7 +161,7 @@
         "movl   %%esi, %3       \n\t"   \
         : "=m" (t), "=m" (c), "=m" (d), "=m" (s)        \
         : "m" (t), "m" (s), "m" (d), "m" (c), "m" (b)   \
-        : "eax", "ecx", "edx", "esi", "edi"             \
+        : "eax", "ebx", "ecx", "edx", "esi", "edi"      \
     );
 #endif /* SSE2 */
 #endif /* i386 */