diff --git a/scripts/mbedtls_dev/bignum_common.py b/scripts/mbedtls_dev/bignum_common.py
index 242217554..5319ec68b 100644
--- a/scripts/mbedtls_dev/bignum_common.py
+++ b/scripts/mbedtls_dev/bignum_common.py
@@ -74,6 +74,10 @@ def combination_pairs(values: List[T]) -> List[Tuple[T, T]]:
     """Return all pair combinations from input values."""
     return [(x, y) for x in values for y in values]
 
+def hex_digits_for_limb(limbs: int, bits_in_limb: int) -> int:
+    """ Retrun the hex digits need for a number of limbs. """
+    return 2 * (limbs * bits_in_limb // 8)
+
 class OperationCommon(test_data_generation.BaseTest):
     """Common features for bignum binary operations.
 
@@ -138,7 +142,7 @@ class OperationCommon(test_data_generation.BaseTest):
 
     @property
     def hex_digits(self) -> int:
-        return 2 * (self.limbs * self.bits_in_limb // 8)
+        return hex_digits_for_limb(self.limbs, self.bits_in_limb)
 
     def format_arg(self, val: str) -> str:
         if self.input_style not in self.input_styles:
diff --git a/scripts/mbedtls_dev/ecp.py b/scripts/mbedtls_dev/ecp.py
index 96ddd057f..c167f6b6f 100644
--- a/scripts/mbedtls_dev/ecp.py
+++ b/scripts/mbedtls_dev/ecp.py
@@ -149,7 +149,9 @@ class EcpP521R1Raw(bignum_common.ModOperationCommon,
 
     @property
     def arg_a(self) -> str:
-        return super().format_arg('{:x}'.format(self.int_a)).zfill(2 * self.hex_digits - 2 * self.bits_in_limb // 8)
+        # Number of limbs: 2 * N - 1
+        hex_digits = bignum_common.hex_digits_for_limb(2 * self.limbs - 1, self.bits_in_limb)
+        return super().format_arg('{:x}'.format(self.int_a)).zfill(hex_digits)
 
     def result(self) -> List[str]:
         result = self.int_a % self.int_n