diff --git a/src/apps/altcp_tls/altcp_tls_mbedtls_mem.c b/src/apps/altcp_tls/altcp_tls_mbedtls_mem.c
index 04d47ae5..9bc7f465 100644
--- a/src/apps/altcp_tls/altcp_tls_mbedtls_mem.c
+++ b/src/apps/altcp_tls/altcp_tls_mbedtls_mem.c
@@ -70,7 +70,7 @@
 
 #if defined(MBEDTLS_PLATFORM_MEMORY) && \
    (!defined(MBEDTLS_PLATFORM_FREE_MACRO) || \
-    defined(MBEDTLS_PLATFORM_CALLOC_MACRO))
+    !defined(MBEDTLS_PLATFORM_CALLOC_MACRO))
 #define ALTCP_MBEDTLS_PLATFORM_ALLOC 1
 #else
 #define ALTCP_MBEDTLS_PLATFORM_ALLOC 0