diff --git a/tests/configs/config-wrapper-malloc-0-null.h b/tests/configs/config-wrapper-malloc-0-null.h
index e7bdbeb..b065c2d 100644
--- a/tests/configs/config-wrapper-malloc-0-null.h
+++ b/tests/configs/config-wrapper-malloc-0-null.h
@@ -21,6 +21,8 @@
 #include "mbedtls/mbedtls_config.h"
 
 #include <stdlib.h>
+
+#ifndef MBEDTLS_PLATFORM_STD_CALLOC
 static inline void *custom_calloc( size_t nmemb, size_t size )
 {
     if( nmemb == 0 || size == 0 )
@@ -30,3 +32,4 @@
 
 #define MBEDTLS_PLATFORM_MEMORY
 #define MBEDTLS_PLATFORM_STD_CALLOC custom_calloc
+#endif