diff --git a/library/bignum_mod.c b/library/bignum_mod.c
index 30289aa..6ff1c4e 100644
--- a/library/bignum_mod.c
+++ b/library/bignum_mod.c
@@ -188,7 +188,7 @@
         return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
 
     mbedtls_mpi_uint *T = mbedtls_calloc( N->limbs * 2 + 1, ciL );
-    if( !T )
+    if( T == NULL )
         return MBEDTLS_ERR_MPI_ALLOC_FAILED;
 
     mbedtls_mpi_mod_raw_mul( X->p, A->p, B->p, N, T );