diff --git a/ChangeLog b/ChangeLog
index ec6410d..6244af3 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -22,6 +22,7 @@
      with non-blocking I/O.
    * Fix compiler warnings on iOS (found by Sander Niemeijer).
    * x509_crt_parse() did not increase total_failed on PEM error
+   * Fix compile error with armcc in mpi_is_prime()
 
 Changes
    * Ciphersuites using SHA-256 or SHA-384 now require TLS 1.x (there is no
diff --git a/library/bignum.c b/library/bignum.c
index 80cf0f8..448e743 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2057,7 +2057,11 @@
                   void *p_rng )
 {
     int ret;
-    const mpi XX = { 1, X->n, X->p }; /* Abs(X) */
+    mpi XX;
+
+    XX.s = 1;
+    XX.n = X->n;
+    XX.p = X->p;
 
     if( mpi_cmp_int( &XX, 0 ) == 0 ||
         mpi_cmp_int( &XX, 1 ) == 0 )