diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index fe821d1..937dcd4 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -642,4 +642,9 @@
 }
 #endif
 
+#ifndef MBEDTLS_TLS_DEFAULT_ALLOW_SHA1
+/* The test infrastructure requires a positive define */
+#define MBEDTLS_X509__DEFAULT_FORBID_SHA1
+#endif
+
 #endif /* mbedtls_x509_crt.h */