diff --git a/lib/vtls/mbedtls.c b/lib/vtls/mbedtls.c
index 56e90acfbc..37e5be2c20 100644
--- a/lib/vtls/mbedtls.c
+++ b/lib/vtls/mbedtls.c
@@ -144,7 +144,10 @@ static void mbed_debug(void *context, int level, const char *f_name,
   (void)level;
   (void)line_nb;
   (void)f_name;
-  size_t len = line ? strlen(line) : 0;
+  size_t len = 0;
+
+  if(line)
+    len = strlen(line);
 
   /* bail out on nowhere to write data
   or if data is empty */