diff --git a/library/ecdsa.c b/library/ecdsa.c
index d57c0c3..afe9558 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -311,7 +311,7 @@
 }
 
 /*
- * Compute and write signature
+ * Compute and write signature. This function assumes that sig is large enough.
  */
 int mbedtls_ecdsa_write_signature( mbedtls_ecdsa_context *ctx, mbedtls_md_type_t md_alg,
                            const unsigned char *hash, size_t hlen,