diff --git a/library/pkwrite.c b/library/pkwrite.c
index bf4ce739e..c95facddc 100644
--- a/library/pkwrite.c
+++ b/library/pkwrite.c
@@ -524,9 +524,19 @@ int mbedtls_pk_write_key_der( mbedtls_pk_context *key, unsigned char *buf, size_
 
 #else /* MBEDTLS_ECP_C */
 
+#if defined(MBEDTLS_USE_TINYCRYPT)
+
+/* see above, replacing ECP_MAX_BYTES with 32 (256-bit) */
+#define ECP_PUB_DER_MAX_BYTES   30 + 2 * 32
+#define ECP_PRV_DER_MAX_BYTES   29 + 3 * 32
+
+#else /* MBEDTLS_USE_TINYCRYPT */
+
 #define ECP_PUB_DER_MAX_BYTES   0
 #define ECP_PRV_DER_MAX_BYTES   0
 
+#endif /* MBEDTLS_USE_TINYCRYPT */
+
 #endif /* MBEDTLS_ECP_C */
 
 #define PUB_DER_MAX_BYTES   RSA_PUB_DER_MAX_BYTES > ECP_PUB_DER_MAX_BYTES ? \