diff --git a/src/core/include/mp-units/ext/prime.h b/src/core/include/mp-units/ext/prime.h
index bb48956b..c734c3d3 100644
--- a/src/core/include/mp-units/ext/prime.h
+++ b/src/core/include/mp-units/ext/prime.h
@@ -28,6 +28,7 @@
 #include <mp-units/ext/algorithm.h>
 
 #ifndef MP_UNITS_IN_MODULE_INTERFACE
+#include <mp-units/ext/contracts.h>
 #ifdef MP_UNITS_IMPORT_STD
 import std;
 #else
@@ -47,8 +48,12 @@ namespace mp_units::detail {
 // Precondition: (a < n).
 // Precondition: (b < n).
 // Precondition: (n > 0).
-[[nodiscard]] consteval uint64_t add_mod(uint64_t a, uint64_t b, uint64_t n)
+[[nodiscard]] consteval std::uint64_t add_mod(std::uint64_t a, std::uint64_t b, std::uint64_t n)
 {
+  MP_UNITS_EXPECTS_DEBUG(a < n);
+  MP_UNITS_EXPECTS_DEBUG(b < n);
+  MP_UNITS_EXPECTS_DEBUG(n > 0u);
+
   if (a >= n - b) {
     return a - (n - b);
   } else {
@@ -61,8 +66,12 @@ namespace mp_units::detail {
 // Precondition: (a < n).
 // Precondition: (b < n).
 // Precondition: (n > 0).
-[[nodiscard]] consteval uint64_t sub_mod(uint64_t a, uint64_t b, uint64_t n)
+[[nodiscard]] consteval std::uint64_t sub_mod(std::uint64_t a, std::uint64_t b, std::uint64_t n)
 {
+  MP_UNITS_EXPECTS_DEBUG(a < n);
+  MP_UNITS_EXPECTS_DEBUG(b < n);
+  MP_UNITS_EXPECTS_DEBUG(n > 0u);
+
   if (a >= b) {
     return a - b;
   } else {
@@ -75,14 +84,18 @@ namespace mp_units::detail {
 // Precondition: (a < n).
 // Precondition: (b < n).
 // Precondition: (n > 0).
-[[nodiscard]] consteval uint64_t mul_mod(uint64_t a, uint64_t b, uint64_t n)
+[[nodiscard]] consteval std::uint64_t mul_mod(std::uint64_t a, std::uint64_t b, std::uint64_t n)
 {
-  if (b == 0u || a < std::numeric_limits<uint64_t>::max() / b) {
+  MP_UNITS_EXPECTS_DEBUG(a < n);
+  MP_UNITS_EXPECTS_DEBUG(b < n);
+  MP_UNITS_EXPECTS_DEBUG(n > 0u);
+
+  if (b == 0u || a < std::numeric_limits<std::uint64_t>::max() / b) {
     return (a * b) % n;
   }
 
-  const uint64_t batch_size = n / a;
-  const uint64_t num_batches = b / batch_size;
+  const std::uint64_t batch_size = n / a;
+  const std::uint64_t num_batches = b / batch_size;
 
   return add_mod(
     // Transform into "negative space" to make the first parameter as small as possible;
@@ -99,15 +112,18 @@ namespace mp_units::detail {
 //
 // Precondition: (a < n).
 // Precondition: (n % 2 == 1).
-[[nodiscard]] consteval uint64_t half_mod_odd(uint64_t a, uint64_t n)
+[[nodiscard]] consteval std::uint64_t half_mod_odd(std::uint64_t a, std::uint64_t n)
 {
+  MP_UNITS_EXPECTS_DEBUG(a < n);
+  MP_UNITS_EXPECTS_DEBUG(n % 2 == 1);
+
   return (a / 2u) + ((a % 2u == 0u) ? 0u : (n / 2u + 1u));
 }
 
 // (base ^ exp) % n.
-[[nodiscard]] consteval uint64_t pow_mod(uint64_t base, uint64_t exp, uint64_t n)
+[[nodiscard]] consteval std::uint64_t pow_mod(std::uint64_t base, std::uint64_t exp, std::uint64_t n)
 {
-  uint64_t result = 1u;
+  std::uint64_t result = 1u;
   base %= n;
 
   while (exp > 0u) {