diff --git a/library/bignum.c b/library/bignum.c
index e52afba..8ed9804 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -273,7 +273,7 @@
     MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, 1 ) );
     memset( X->p, 0, X->n * ciL );
 
-    X->p[0] = ( z < 0 ) ? -z : z;
+    X->p[0] = ( z < 0 ) ? -(mbedtls_mpi_uint)z : z;
     X->s    = ( z < 0 ) ? -1 : 1;
 
 cleanup:
@@ -1093,7 +1093,7 @@
     mbedtls_mpi_uint p[1];
     MPI_VALIDATE_RET( X != NULL );
 
-    *p  = ( z < 0 ) ? -z : z;
+    *p  = ( z < 0 ) ? -(mbedtls_mpi_uint)z : z;
     Y.s = ( z < 0 ) ? -1 : 1;
     Y.n = 1;
     Y.p = p;
@@ -1334,7 +1334,7 @@
     MPI_VALIDATE_RET( X != NULL );
     MPI_VALIDATE_RET( A != NULL );
 
-    p[0] = ( b < 0 ) ? -b : b;
+    p[0] = ( b < 0 ) ? -(mbedtls_mpi_uint)b : b;
     B.s = ( b < 0 ) ? -1 : 1;
     B.n = 1;
     B.p = p;
@@ -1352,7 +1352,7 @@
     MPI_VALIDATE_RET( X != NULL );
     MPI_VALIDATE_RET( A != NULL );
 
-    p[0] = ( b < 0 ) ? -b : b;
+    p[0] = ( b < 0 ) ? -(mbedtls_mpi_uint)b : b;
     B.s = ( b < 0 ) ? -1 : 1;
     B.n = 1;
     B.p = p;
@@ -1773,7 +1773,7 @@
     mbedtls_mpi_uint p[1];
     MPI_VALIDATE_RET( A != NULL );
 
-    p[0] = ( b < 0 ) ? -b : b;
+    p[0] = ( b < 0 ) ? -(mbedtls_mpi_uint)b : b;
     B.s = ( b < 0 ) ? -1 : 1;
     B.n = 1;
     B.p = p;