diff --git a/ChangeLog b/ChangeLog
index 16b2f84c7..4ae771b05 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,7 @@ Features
   * Support for DTLS 1.0 and 1.2 (RFC 6347).
 
 API Changes
+   * Signature of mpi_mul_mpi() changed to make the last argument unsigned
    * Remove the PBKDF2 module (use PKCS5).
    * Remove POLARSSL_ERROR_STRERROR_BC (use mbedtls_strerror()).
    * Headers are now found in the 'mbedtls' directory (previously 'polarssl').
diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index df25bd1f1..44248f360 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -571,18 +571,17 @@ int mpi_mul_mpi( mpi *X, const mpi *A, const mpi *B );
 
 /**
  * \brief          Baseline multiplication: X = A * b
- *                 Note: despite the functon signature, b is treated as a
- *                 t_uint.  Negative values of b are treated as large positive
- *                 values.
  *
  * \param X        Destination MPI
  * \param A        Left-hand MPI
- * \param b        The integer value to multiply with
+ * \param b        The unsigned integer value to multiply with
+ *
+ * \note           b is unsigned
  *
  * \return         0 if successful,
  *                 POLARSSL_ERR_MPI_MALLOC_FAILED if memory allocation failed
  */
-int mpi_mul_int( mpi *X, const mpi *A, t_sint b );
+int mpi_mul_int( mpi *X, const mpi *A, t_uint b );
 
 /**
  * \brief          Division by mpi: A = Q * B + R
diff --git a/library/bignum.c b/library/bignum.c
index 86376a470..2d04eb46e 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1170,7 +1170,7 @@ cleanup:
 /*
  * Baseline multiplication: X = A * b
  */
-int mpi_mul_int( mpi *X, const mpi *A, t_sint b )
+int mpi_mul_int( mpi *X, const mpi *A, t_uint b )
 {
     mpi _B;
     t_uint p[1];