diff --git a/programs/pkey/pk_verify.c b/programs/pkey/pk_verify.c
index a6bfe3f..72caf71 100644
--- a/programs/pkey/pk_verify.c
+++ b/programs/pkey/pk_verify.c
@@ -65,7 +65,7 @@
     size_t i;
     mbedtls_pk_context pk;
     unsigned char hash[32];
-    unsigned char buf[MBEDTLS_MPI_MAX_SIZE];
+    unsigned char buf[MBEDTLS_PK_SIGNATURE_MAX_SIZE];
     char filename[512];
 
     mbedtls_pk_init( &pk );