diff --git a/library/pkcs11.c b/library/pkcs11.c
index 172ffc1..b4f4787 100644
--- a/library/pkcs11.c
+++ b/library/pkcs11.c
@@ -116,7 +116,7 @@
                        int mode, int *olen,
                        const unsigned char *input,
                        unsigned char *output,
-                       int output_max_len )
+                       unsigned int output_max_len )
 {
     size_t input_len, output_len;