diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index da18599..f832db5 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -69,7 +69,7 @@
  */
 #define POLARSSL_MPI_MAX_LIMBS                             10000
 
-#if !defined(POLARSSL_CONFIG_OPTIONS)
+#if !defined(POLARSSL_MPI_WINDOW_SIZE)
 /*
  * Maximum window size used for modular exponentiation. Default: 6
  * Minimum value: 1. Maximum value: 6.
@@ -80,7 +80,9 @@
  * Reduction in size, reduces speed.
  */
 #define POLARSSL_MPI_WINDOW_SIZE                           6        /**< Maximum windows size used. */
+#endif /* !POLARSSL_MPI_WINDOW_SIZE */
 
+#if !defined(POLARSSL_MPI_MAX_SIZE)
 /*
  * Maximum size of MPIs allowed in bits and bytes for user-MPIs.
  * ( Default: 512 bytes => 4096 bits, Maximum tested: 2048 bytes => 16384 bits )
@@ -89,8 +91,7 @@
  * of limbs required (POLARSSL_MPI_MAX_LIMBS) is higher.
  */
 #define POLARSSL_MPI_MAX_SIZE                              512      /**< Maximum number of bytes for usable MPIs. */
-
-#endif /* !POLARSSL_CONFIG_OPTIONS */
+#endif /* !POLARSSL_MPI_MAX_SIZE */
 
 #define POLARSSL_MPI_MAX_BITS                              ( 8 * POLARSSL_MPI_MAX_SIZE )    /**< Maximum number of bits for usable MPIs. */