diff --git a/scripts/build_ffi.py b/scripts/build_ffi.py
index 159323e..18caa3a 100644
--- a/scripts/build_ffi.py
+++ b/scripts/build_ffi.py
@@ -559,6 +559,7 @@ def build_ffi(local_wolfssl, features):
 
         int mp_init (mp_int * a);
         int mp_to_unsigned_bin (mp_int * a, unsigned char *b);
+        int mp_to_unsigned_bin_len (mp_int * a, unsigned char *b, int c);
         int mp_read_unsigned_bin (mp_int * a, const unsigned char *b, int c);
         """
 
diff --git a/wolfcrypt/ciphers.py b/wolfcrypt/ciphers.py
index eb3f27f..d71c847 100644
--- a/wolfcrypt/ciphers.py
+++ b/wolfcrypt/ciphers.py
@@ -1390,11 +1390,11 @@ def sign_raw(self, plaintext, rng=Random()):
                 if ret != 0:  # pragma: no cover
                     raise WolfCryptError("Signature error (%d)" % ret)
 
-                ret = _lib.mp_to_unsigned_bin(R, R_bin)
+                ret = _lib.mp_to_unsigned_bin_len(R, R_bin, self.size)
                 if ret != 0:  # pragma: no cover
                     raise WolfCryptError("wolfCrypt error (%d)" % ret)
 
-                ret = _lib.mp_to_unsigned_bin(S, S_bin)
+                ret = _lib.mp_to_unsigned_bin_len(S, S_bin, self.size)
                 if ret != 0:  # pragma: no cover
                     raise WolfCryptError("wolfCrypt error (%d)" % ret)