diff --git a/library/bignum.c b/library/bignum.c
index 7841bea433..07e099afbc 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -59,8 +59,8 @@
 #endif
 
 /* Implementation that should never be optimized out by the compiler */
-static void mbedtls_zeroize( void *v, size_t n ) {
-    volatile unsigned char *p = v; while( n-- ) *p++ = 0;
+static void mbedtls_zeroize( mbedtls_mpi_uint *v, size_t n ) {
+    volatile mbedtls_mpi_uint *p = v; while( n-- ) *p++ = 0;
 }
 
 #define ciL    (sizeof(mbedtls_mpi_uint))         /* chars in limb  */
@@ -99,7 +99,7 @@ void mbedtls_mpi_free( mbedtls_mpi *X )
 
     if( X->p != NULL )
     {
-        mbedtls_zeroize( X->p, X->n * ciL );
+        mbedtls_zeroize( X->p, X->n );
         mbedtls_free( X->p );
     }
 
@@ -126,7 +126,7 @@ int mbedtls_mpi_grow( mbedtls_mpi *X, size_t nblimbs )
         if( X->p != NULL )
         {
             memcpy( p, X->p, X->n * ciL );
-            mbedtls_zeroize( X->p, X->n * ciL );
+            mbedtls_zeroize( X->p, X->n );
             mbedtls_free( X->p );
         }
 
@@ -164,7 +164,7 @@ int mbedtls_mpi_shrink( mbedtls_mpi *X, size_t nblimbs )
     if( X->p != NULL )
     {
         memcpy( p, X->p, i * ciL );
-        mbedtls_zeroize( X->p, X->n * ciL );
+        mbedtls_zeroize( X->p, X->n );
         mbedtls_free( X->p );
     }