diff --git a/fpga/lib/ip.cpp b/fpga/lib/ip.cpp
index 63cffdebc..0a21d16e3 100644
--- a/fpga/lib/ip.cpp
+++ b/fpga/lib/ip.cpp
@@ -223,8 +223,8 @@ IpCoreFactory::make(PCIeCard* card, json_t *json_ips)
 					json_t* json_block;
 					json_object_foreach(json_instance, block_name, json_block) {
 
-						unsigned int base, high, size;
-						int ret = json_unpack(json_block, "{ s: i, s: i, s: i }",
+						json_int_t base, high, size;
+						int ret = json_unpack(json_block, "{ s: I, s: I, s: I }",
 						                      "baseaddr", &base,
 						                      "highaddr", &high,
 						                      "size", &size);