diff --git a/library/bignum.c b/library/bignum.c
index 4536a3b..4c99e04 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -120,7 +120,7 @@
 
     if( X->n < nblimbs )
     {
-        if( ( p = mbedtls_calloc( nblimbs, ciL ) ) == NULL )
+        if( ( p = (mbedtls_mpi_uint*)mbedtls_calloc( nblimbs, ciL ) ) == NULL )
             return( MBEDTLS_ERR_MPI_ALLOC_FAILED );
 
         if( X->p != NULL )
@@ -158,7 +158,7 @@
     if( i < nblimbs )
         i = nblimbs;
 
-    if( ( p = mbedtls_calloc( i, ciL ) ) == NULL )
+    if( ( p = (mbedtls_mpi_uint*)mbedtls_calloc( i, ciL ) ) == NULL )
         return( MBEDTLS_ERR_MPI_ALLOC_FAILED );
 
     if( X->p != NULL )