diff --git a/include/polarssl/pkcs11.h b/include/polarssl/pkcs11.h
index efe7f14..5087e19 100644
--- a/include/polarssl/pkcs11.h
+++ b/include/polarssl/pkcs11.h
@@ -97,7 +97,7 @@
                        int mode, int *olen,
                        const unsigned char *input,
                        unsigned char *output,
-                       int output_max_len );
+                       unsigned int output_max_len );
 
 /**
  * \brief          Do a private RSA to sign a message digest