diff --git a/libraries/AP_Param/AP_Param.cpp b/libraries/AP_Param/AP_Param.cpp
index f4f1147be8..f70ffdf997 100644
--- a/libraries/AP_Param/AP_Param.cpp
+++ b/libraries/AP_Param/AP_Param.cpp
@@ -623,7 +623,7 @@ uint8_t AP_Param::type_size(enum ap_var_type type)
     case AP_PARAM_VECTOR3F:
         return 3*4;
     }
-    Debug("unknown type %u\n", type);
+    Debug("unknown type %d\n", type);
     return 0;
 }