diff --git a/libguile/r6rs-ports.c b/libguile/r6rs-ports.c
index e944c7aab..00eea04a8 100644
--- a/libguile/r6rs-ports.c
+++ b/libguile/r6rs-ports.c
@@ -408,11 +408,11 @@ SCM_DEFINE (scm_get_bytevector_n, "get-bytevector-n", 2, 0, 0,
 #define FUNC_NAME s_scm_get_bytevector_n
 {
   SCM result;
-  unsigned c_count;
+  size_t c_count;
   size_t c_read;
 
   SCM_VALIDATE_BINARY_INPUT_PORT (1, port);
-  c_count = scm_to_uint (count);
+  c_count = scm_to_size_t (count);
 
   result = scm_c_make_bytevector (c_count);
 
@@ -444,13 +444,13 @@ SCM_DEFINE (scm_get_bytevector_n_x, "get-bytevector-n!", 4, 0, 0,
 #define FUNC_NAME s_scm_get_bytevector_n_x
 {
   SCM result;
-  unsigned c_start, c_count, c_len;
+  size_t c_start, c_count, c_len;
   size_t c_read;
 
   SCM_VALIDATE_BINARY_INPUT_PORT (1, port);
   SCM_VALIDATE_BYTEVECTOR (2, bv);
-  c_start = scm_to_uint (start);
-  c_count = scm_to_uint (count);
+  c_start = scm_to_size_t (start);
+  c_count = scm_to_size_t (count);
 
   c_len = SCM_BYTEVECTOR_LENGTH (bv);