diff --git a/ChangeLog b/ChangeLog
index b56867c..afa56d1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,6 +5,8 @@
 Bugfix
    * Fix compilation error on C++, because of a variable named new.
      Found and fixed by Hirotaka Niisato in #1783.
+   * Fix the inline assembly for the MPI multiply helper function for i386 and
+     i386 with SSE2. Found by László Langó. Fixes #1550
    * Fix a memory leak in mbedtls_x509_csr_parse(), found by catenacyber,
      Philippe Antoine. Fixes #1623.
    * Clarify documentation for mbedtls_ssl_write() to include 0 as a valid
diff --git a/include/mbedtls/bn_mul.h b/include/mbedtls/bn_mul.h
index 7f8eb1a..8d799ec 100644
--- a/include/mbedtls/bn_mul.h
+++ b/include/mbedtls/bn_mul.h
@@ -48,7 +48,14 @@
 /* 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(                                    \
@@ -141,7 +148,7 @@
         "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
@@ -153,7 +160,7 @@
         "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 */