diff --git a/ChangeLog b/ChangeLog
index f0be79ed6..0293eb33c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -48,6 +48,11 @@ Bugfix
    * Improve code clarity in x509_crt module, removing false-positive
      uninitialized variable warnings on some recent toolchains (GCC8, etc).
      Discovered and fixed by Andy Gross (Linaro), #2392.
+   * Zero length buffer check for undefined behavior in
+     mbedtls_platform_zeroize(). Fixes ARMmbed/mbed-crypto#49.
+   * Fix bug in endianness conversion in bignum module. This lead to
+     functionally incorrect code on bigendian systems which don't have
+     __BYTE_ORDER__ defined. Reported by Brendan Shanks. Fixes #2622.
 
 Changes
    * Make it easier to define MBEDTLS_PARAM_FAILED as assert (which config.h
diff --git a/library/bignum.c b/library/bignum.c
index c6feada8b..d1717e943 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -742,10 +742,15 @@ cleanup:
 static mbedtls_mpi_uint mpi_uint_bigendian_to_host_c( mbedtls_mpi_uint x )
 {
     uint8_t i;
+    unsigned char *x_ptr;
     mbedtls_mpi_uint tmp = 0;
-    /* This works regardless of the endianness. */
-    for( i = 0; i < ciL; i++, x >>= 8 )
-        tmp |= ( x & 0xFF ) << ( ( ciL - 1 - i ) << 3 );
+
+    for( i = 0, x_ptr = (unsigned char*) &x; i < ciL; i++, x_ptr++ )
+    {
+        tmp <<= CHAR_BIT;
+        tmp |= (mbedtls_mpi_uint) *x_ptr;
+    }
+
     return( tmp );
 }
 
diff --git a/library/platform_util.c b/library/platform_util.c
index 756e22679..b1f745097 100644
--- a/library/platform_util.c
+++ b/library/platform_util.c
@@ -72,7 +72,10 @@ static void * (* const volatile memset_func)( void *, int, size_t ) = memset;
 
 void mbedtls_platform_zeroize( void *buf, size_t len )
 {
-    memset_func( buf, 0, len );
+    MBEDTLS_INTERNAL_VALIDATE( len == 0 || buf != NULL );
+
+    if( len > 0 )
+        memset_func( buf, 0, len );
 }
 #endif /* MBEDTLS_PLATFORM_ZEROIZE_ALT */
 
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index 238ed97d0..fa334c38b 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -420,9 +420,9 @@ has_mem_err() {
     fi
 }
 
-# Wait for process $2 to be listening on port $1
+# Wait for process $2 named $3 to be listening on port $1. Print error to $4.
 if type lsof >/dev/null 2>/dev/null; then
-    wait_server_start() {
+    wait_app_start() {
         START_TIME=$(date +%s)
         if [ "$DTLS" -eq 1 ]; then
             proto=UDP
@@ -432,8 +432,8 @@ if type lsof >/dev/null 2>/dev/null; then
         # Make a tight loop, server normally takes less than 1s to start.
         while ! lsof -a -n -b -i "$proto:$1" -p "$2" >/dev/null 2>/dev/null; do
               if [ $(( $(date +%s) - $START_TIME )) -gt $DOG_DELAY ]; then
-                  echo "SERVERSTART TIMEOUT"
-                  echo "SERVERSTART TIMEOUT" >> $SRV_OUT
+                  echo "$3 START TIMEOUT"
+                  echo "$3 START TIMEOUT" >> $4
                   break
               fi
               # Linux and *BSD support decimal arguments to sleep. On other
@@ -442,12 +442,22 @@ if type lsof >/dev/null 2>/dev/null; then
         done
     }
 else
-    echo "Warning: lsof not available, wait_server_start = sleep"
-    wait_server_start() {
+    echo "Warning: lsof not available, wait_app_start = sleep"
+    wait_app_start() {
         sleep "$START_DELAY"
     }
 fi
 
+# Wait for server process $2 to be listening on port $1.
+wait_server_start() {
+    wait_app_start $1 $2 "SERVER" $SRV_OUT
+}
+
+# Wait for proxy process $2 to be listening on port $1.
+wait_proxy_start() {
+    wait_app_start $1 $2 "PROXY" $PXY_OUT
+}
+
 # Given the client or server debug output, parse the unix timestamp that is
 # included in the first 4 bytes of the random bytes and check that it's within
 # acceptable bounds
@@ -600,7 +610,7 @@ run_test() {
             echo "$PXY_CMD" > $PXY_OUT
             $PXY_CMD >> $PXY_OUT 2>&1 &
             PXY_PID=$!
-            # assume proxy starts faster than server
+            wait_proxy_start "$PXY_PORT" "$PXY_PID"
         fi
 
         check_osrv_dtls