diff --git a/src/hpc/thread.c b/src/hpc/thread.c
index 60b7ff07e3..2a36cc7c1c 100644
--- a/src/hpc/thread.c
+++ b/src/hpc/thread.c
@@ -1309,12 +1309,21 @@ Region * CurrentRegion(void)
     return TLS(currentRegion);
 }
 
+// Kernel debugging information for guards.
+//
+// When compiled with -DDEBUG_GUARDS, the GAP variable GUARD_ERROR_STACK
+// will contain the C stack after an error, allowing us to located where
+// it occurred without having to fire up a debugger first.
+//
+// The variables NumReadErrors and NumWriteErrors track the number of
+// failed guard checks. These are used in conjunction with the low-level
+// function DISABLE_GUARDS() in order to track how many guard failures
+// occur in a given section of code.
+
 #ifdef DEBUG_GUARDS
 extern GVarDescriptor GUARD_ERROR_STACK;
 #endif
 
-// These are temporary debugging functions.
-
 static Int NumReadErrors = 0;
 static Int NumWriteErrors = 0;
 
diff --git a/src/system.h b/src/system.h
index f88c077f88..23cec238a2 100644
--- a/src/system.h
+++ b/src/system.h
@@ -63,10 +63,12 @@ GAP_STATIC_ASSERT(sizeof(void *) == SIZEOF_VOID_P, "sizeof(void *) is wrong");
 #endif
 #endif
 
-// If we are not running HPC-GAP, disable read and write guards.
+// If we are not running HPC-GAP, guards should be disabled
 
 #ifndef HPCGAP
-#undef USE_HPC_GUARDS
+#ifdef USE_HPC_GUARDS
+#error Do not use --enable-guards without --enable-hpcgap.
+#endif
 #endif
 
 /****************************************************************************