diff --git a/library/bignum.c b/library/bignum.c
index 8b9082cdc..e9ac56505 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1614,7 +1614,7 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
     mbedtls_mpi RR, T, W[ 2 << MBEDTLS_MPI_WINDOW_SIZE ], Apos;
     int neg;
 
-    if( mbedtls_mpi_cmp_int( N, 0 ) < 0 || ( N->p[0] & 1 ) == 0 )
+    if( mbedtls_mpi_cmp_int( N, 0 ) <= 0 || ( N->p[0] & 1 ) == 0 )
         return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
 
     if( mbedtls_mpi_cmp_int( E, 0 ) < 0 )