diff --git a/library/bignum.c b/library/bignum.c
index dfe976d64..6d0a67cb8 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2044,7 +2044,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
     size_t i, j, nblimbs;
     size_t bufsize, nbits;
     mbedtls_mpi_uint ei, mm, state;
-    mbedtls_mpi RR, T, W[ 2 << MBEDTLS_MPI_WINDOW_SIZE ], Apos;
+    mbedtls_mpi RR, T, W[ 1 << MBEDTLS_MPI_WINDOW_SIZE ], Apos;
     int neg;
 
     MPI_VALIDATE_RET( X != NULL );