diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index cff22eaa8..2fb86c7eb 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -128,28 +128,32 @@ extern "C" {
 void mbedtls_strerror( int errnum, char *buffer, size_t buflen );
 
 /**
- * \brief Translate high level part of a mbed TLS error code into a string
+ * \brief Translate the high-level part of an Mbed TLS error code into a string
  *        representation.
  *
  * This function returns a const pointer to an un-modifiable string. The caller
- * must not try to modify the string and use it only for logging purposes.
+ * must not try to modify the string. It is intended to be used mostly for
+ * logging purposes.
  *
  * \param error_code    error code
  *
- * \return The string representation of the error code.
+ * \return The string representation of the error code, or \c NULL if the error
+ *         code is unknown.
  */
 const char * mbedtls_high_level_strerr( int error_code );
 
 /**
- * \brief Translate low level part of a mbed TLS error code into a string
+ * \brief Translate the low-level part of an Mbed TLS error code into a string
  *        representation.
  *
  * This function returns a const pointer to an un-modifiable string. The caller
- * must not try to modify the string and use it only for logging purposes.
+ * must not try to modify the string. It is intended to be used mostly for
+ * logging purposes.
  *
  * \param error_code    error code
  *
- * \return The string representation of the error code.
+ * \return The string representation of the error code, or \c NULL if the error
+ *         code is unknown.
  */
 const char * mbedtls_low_level_strerr( int error_code );
 
diff --git a/library/error.c b/library/error.c
index ee1f8b704..38b658006 100644
--- a/library/error.c
+++ b/library/error.c
@@ -215,9 +215,16 @@
 
 const char * mbedtls_high_level_strerr( int error_code )
 {
+    int high_level_error_code;
     const char *error_description = NULL;
 
-    switch( error_code )
+    if( error_code < 0 )
+        error_code = -error_code;
+
+    /* Extract the high-level part from the error code. */
+    high_level_error_code = error_code & 0xFF80;
+
+    switch( high_level_error_code )
     {
         /* Begin Auto-Generated Code. */
 #if defined(MBEDTLS_CIPHER_C)
@@ -725,9 +732,16 @@ const char * mbedtls_high_level_strerr( int error_code )
 
 const char * mbedtls_low_level_strerr( int error_code )
 {
+    int low_level_error_code;
     const char *error_description = NULL;
 
-    switch( error_code )
+    if( error_code < 0 )
+        error_code = -error_code;
+
+    /* Extract the low-level part from the error code. */
+    low_level_error_code = error_code & ~0xFF80;
+
+    switch( low_level_error_code )
     {
         /* Begin Auto-Generated Code. */
 #if defined(MBEDTLS_AES_C)
@@ -1154,7 +1168,7 @@ void mbedtls_strerror( int ret, char *buf, size_t buflen )
         use_ret = ret & 0xFF80;
 
         // Translate high level error code.
-        high_level_error_description = mbedtls_high_level_strerr(use_ret);
+        high_level_error_description = mbedtls_high_level_strerr( ret );
 
         if( high_level_error_description == NULL )
             mbedtls_snprintf( buf, buflen, "UNKNOWN ERROR CODE (%04X)", use_ret );
@@ -1191,7 +1205,7 @@ void mbedtls_strerror( int ret, char *buf, size_t buflen )
     }
 
     // Translate low level error code.
-    low_level_error_description = mbedtls_low_level_strerr( use_ret );
+    low_level_error_description = mbedtls_low_level_strerr( ret );
 
     if( low_level_error_description == NULL )
         mbedtls_snprintf( buf, buflen, "UNKNOWN ERROR CODE (%04X)", use_ret );
diff --git a/scripts/data_files/error.fmt b/scripts/data_files/error.fmt
index 5699d9ea2..0e128e84f 100644
--- a/scripts/data_files/error.fmt
+++ b/scripts/data_files/error.fmt
@@ -44,9 +44,16 @@ HEADER_INCLUDED
 
 const char * mbedtls_high_level_strerr( int error_code )
 {
+    int high_level_error_code;
     const char *error_description = NULL;
 
-    switch( error_code )
+    if( error_code < 0 )
+        error_code = -error_code;
+
+    /* Extract the high-level part from the error code. */
+    high_level_error_code = error_code & 0xFF80;
+
+    switch( high_level_error_code )
     {
         /* Begin Auto-Generated Code. */
 HIGH_LEVEL_CODE_CHECKS
@@ -61,9 +68,16 @@ HIGH_LEVEL_CODE_CHECKS
 
 const char * mbedtls_low_level_strerr( int error_code )
 {
+    int low_level_error_code;
     const char *error_description = NULL;
 
-    switch( error_code )
+    if( error_code < 0 )
+        error_code = -error_code;
+
+    /* Extract the low-level part from the error code. */
+    low_level_error_code = error_code & ~0xFF80;
+
+    switch( low_level_error_code )
     {
         /* Begin Auto-Generated Code. */
 LOW_LEVEL_CODE_CHECKS
@@ -96,7 +110,7 @@ void mbedtls_strerror( int ret, char *buf, size_t buflen )
         use_ret = ret & 0xFF80;
 
         // Translate high level error code.
-        high_level_error_description = mbedtls_high_level_strerr(use_ret);
+        high_level_error_description = mbedtls_high_level_strerr( ret );
 
         if( high_level_error_description == NULL )
             mbedtls_snprintf( buf, buflen, "UNKNOWN ERROR CODE (%04X)", use_ret );
@@ -133,7 +147,7 @@ void mbedtls_strerror( int ret, char *buf, size_t buflen )
     }
 
     // Translate low level error code.
-    low_level_error_description = mbedtls_low_level_strerr( use_ret );
+    low_level_error_description = mbedtls_low_level_strerr( ret );
 
     if( low_level_error_description == NULL )
         mbedtls_snprintf( buf, buflen, "UNKNOWN ERROR CODE (%04X)", use_ret );