diff --git a/tests/configs/config-wrapper-malloc-0-null.h b/tests/configs/config-wrapper-malloc-0-null.h
index 69a9578..add1a78 100644
--- a/tests/configs/config-wrapper-malloc-0-null.h
+++ b/tests/configs/config-wrapper-malloc-0-null.h
@@ -24,6 +24,8 @@
 #include "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 )
@@ -33,5 +35,6 @@
 
 #define MBEDTLS_PLATFORM_MEMORY
 #define MBEDTLS_PLATFORM_STD_CALLOC custom_calloc
+#endif
 
 #endif /* MBEDTLS_CONFIG_H */